Deduction of Mechanismsf0b7d1a0-e882-4e69-9f9...∆S ≠ =4.58(logA −13.2) 06.05.2020...
105
Transition state theory (TST): /RT E a e A k − ⋅ = /RT H /R S B e e h T k k ≠ ≠ ∆ − ∆ ⋅ ⋅ ⋅ = χ χ = Übergangsfaktor, generally = 1 k B : Boltzmann h: Planck A and E a are considered T independent, mostly correct over a small range of ∆T 06.05.2020 CHE323-FS20-T2-1 Deduction of Mechanisms 4.2 Transition State Theory Observations: Reaction rates increase almost ever with increasing T this relation k vs E a is consistent with the Arrhenius equation k describes the overall reaction rate "constant"
Transcript of Deduction of Mechanismsf0b7d1a0-e882-4e69-9f9...∆S ≠ =4.58(logA −13.2) 06.05.2020...
Transition state theory (TST)
RTEaeAk minussdot=
RTHRSB eeh
Tkknene ∆minus∆ sdotsdot
sdot= χ
χ = Uumlbergangsfaktor generally = 1 kB Boltzmann h Planck
A and Ea are considered T independent mostly correct over a small range of ∆T
06052020 CHE323-FS20-T2-1
Deduction of Mechanisms
42 Transition State Theory
Observations Reaction rates increase almost ever with increasing T
this relation k vs Ea is consistent with the Arrhenius equation
k describes the overall reaction rate constant
RTElnAlnk aminus= plot lnk vs 1T liefert A und Ea
ln(kT) vs T-1 yields ∆Hne (slope) and ∆Sne with ln(kBh) = 2376
26 kJmol bei RTand 132)458(logAS minus=∆ ne
06052020 CHE323-FS20-T2-2
Deduction of Mechanisms
42 Transition State Theory
Arrhenius
translates into TST kTln = kB
hln + ∆Sne
R∆Hne
RsdotT-
relation between Arrhenius and TST
the activation parameters ∆Hne and ∆Sne are very indicative for the mechanism
∆Hne = RmiddotTmiddotlnT
06052020 CHE323-FS20-T2-3
Deduction of Mechanisms
42 Transition State Theory
Example CoOC
OC PR3
PR3
CO
CO
COCo
OC
OC CO
PR3
CO
CO
2+
+ PR3
∆Hne = 37 kcalmol ∆Sne = + 25 eu
PRTVlnklnk 0
ne∆minus=
relates to volume of activation ∆Vne
thermodynamics dG = -SsdotdT + Vsdotdp kinetics
lnk depends linearly on the pressure
lnk (or logk) vs p gives a straight line with ∆Vne as slope
assuming that ∆Vne is pressure independent
what does this tell us with respect to mechanism
06052020 CHE323-FS20-T2-4
Deduction of Mechanisms
42 Transition State Theory
it gives an insight into the intimate mechanism of the rds
ground state transition states
D Id IaI A
∆Sne ++ + 0 - --∆Vne ++ + 0 - --
D and Id and A and Ia are often difficult to differentiate
∆Sne and ∆V ne give us a clear insight into the mechanism eg ligand exchange
06052020 CHE323-FS20-T2-5
Deduction of Mechanisms
42 Transition State Theory
interpretation of ∆Sne
interpretation of ∆Vne
larr associative
larr dissociative
∆Vdeg (H2O) = 18 cm3 mol
∆Vne = - 15 cm3 mol rArr A ndash process= + 15 cm3 mol rArr D ndash process
ligand self exchange
∆Vne represents the sum of the partial mol volumina in the transitions state
06052020 CHE323-FS20-T2-6
[M(OH2)6]n+
kex
[M(OH2)5(OH2)]n+ + H2O
bdquono reaction reaction ∆Go = O
for D processes k = kex
kex covers an extremely broad range
Deduction of Mechanisms
42 Transition State Theory
+ OH2
06052020 CHE323-FS20-T2-7
[M(OH2)6]n+Ka
[M(OH2)5(OH)] + H+
v = kex [M-OH] = kex middot Ka [M-OH2] [H]+
kex H+
[M(OH2)6]n+
∆H
(kJmiddotmol-1)∆S (eu) ∆V
(cm3sdotmol-1)Assigned mechanism
Ga3+ 67 +30 +5 Id
Ga(OH)2+ +6 Id
Ti3+ 43 -12 +1 Ia
Cr3+ 109 +12 -10 Ia
Cr(OH)2+ +3 I
Fe3+ 65 +12 -5 Ia
Fe(OH)2+ +7 Id
Ru3+ 90 -48 -8 Ia
Ru(OH)2+ +1 I
Rh3+ 131 +29 -4 Ia
Rh(OH)2+ +2 I
Ir3+ 131 +2 -6 Ia
Ir(OH)2+ +1 I
Deduction of Mechanisms
42 Transition State Theory
water self exchange rates indicative for substitution mechanisms in general
kex H2O
[M(OH2)6]n+
AH+ H+ + AKa A + B Pk2
][H][B][AHKkv a2 +
+
=
∆minus
∆sdot
∆minus
∆=
nene
RTHexp
RSexp
RTHexp
RSexp
hTkk
θa
θa22B
∆minus∆minus
∆+∆=
nene
RTHHexp
RSSexp
hTkk
θa2
θa2B
06052020 CHE323-FS20-T2-8
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
lets look at a pH dependent scheme
pm
the composed rate constant is k2middotKA
applying TST and vant Hoff
steady state System A + B Ik3
k-3I Pk4
k-3 gtgt k4 then [A][B]k
kkv3
43
minus
=
06052020 CHE323-FS20-T2-9
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
linearization we do not get ∆Hne but -∆Hne - ∆Hθ
we must know pre-equilibrium (Ka in this case) to get the true ∆Hne
Question find a situation for which ∆Gne is negative
eg the reaction becomes slower with increasing temperature
∆+∆minus∆minus
∆+∆minus∆=
neneminus
neneneminus
ne
RTHHHexp
RSSSexp
hTkk 433433B
06052020 CHE323-FS20-T2-10
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
we also get a linear profile the parameters are composed of the individual values
deviations from linearity in practice
a typical Eyring (or Arrhenius) plot
lnk
T
1T
slope yields ∆Hne
intercept yields ∆Sne
06052020 CHE323-FS20-T2-11
Deduction of Mechanisms
42 Transition State Theory
lnk
T
1T
lnk
T
1T
but sometimes the straight line looks rather like this
or like this hellip what does that mean
non-linearity often even in professional presentations still considered as linearhellip
06052020 CHE323-FS20-T2-12
A
P1
P2
k1
k2
)[A]k(kk[A]dt
d[A]21 +==minus
∆minus
∆+
∆minus
∆=
nenenene
RTHexp
RSexp
RTHexp
RSexp
hTkk 2211B
Deduction of Mechanisms
42 Transition State Theory
concurrent reaction schemes will result in non-linearity
what about a simple consecutive scheme like this one
A + B ABk1
k-1AB + C E + F
k2
06052020 CHE323-FS20-T2-13
the T profile of such a scheme is non-linear
Deduction of Mechanisms
42 Transition State Theory
it can however be dissected in two linear profiles
06052020 CHE323-FS20-T2-14
Deduction of Mechanisms
42 Transition State Theoryan simple example from practice
MH3N
H3N NH3
ONO
NH3
NH3
2+
MH3N
H3N NH3
NO2
NH3
NH3
2+
a linkage isomerization
activation parameters
Co Rh Ir
∆Sne -17+-3 -33+-7 -11+-4 eu
∆Hne 92 80 95 kJmol
∆Vne -67 -74 -59 cm3mol
is there an isokinetic relationship
what mechanism of isomerization do you suggest
The reaction mechanisms for the forward and the backward reaction
are identical they are mirror images of each other
A I I Pk1
k-1
k2
k-206052020 CHE323-FS20-T2-15
Deduction of Mechanisms
43 Microscopic Reversibility
This principle is always valid
even when multiple steps equilibria or branched reactions are involved
lets look at the simple reaction
if it were not valid we would run into problems with thermodynamics
[Fe(OH2)6]3+ + NCS- [Fe(NCS)(OH2)5]2+ + H2O
is pH dependent and can be described with
][FeNCS][H
kk]][NCS[Fe][H
kkdt
]d[FeNCS 221
321
2+
+minus
minusminus+
+
+
+minus
+=
0dt
]d[FeNCS2=
+
][Hkk
][Hkk
][NCS]O)[Fe(H]FeNCSO)[(HK
21
21
ee362
e2
52
+minus
minus
+
minus+
+
+
+==
06052020 CHE323-FS20-T2-16
Deduction of Mechanisms
43 Microscopic Reversibility
the rate law
forward reaction backward reaction
after equilibrium is achieved
and K becomes pH dependent
which it is obviously not according to reaction equation
e2
1ee3
1 ][FeNCSk][NCS][Fek +minus
minus+ =
e
e2
2e
ee3
2 ][H][FeNCSk
][H][NCS][Fek +
+
minus+
minus+
=
2
2
1
1
kk
kkK
minusminus
==
06052020 CHE323-FS20-T2-17
Deduction of Mechanisms
43 Microscopic Reversibility
Without consideration of the individual steps pH dependence
conflict with thermodynamics
the problem is solved by considering the microscopic reversibility of all
individual equilibria involved separately
pH independent path
pH dependent path
hence as it should be
06052020 CHE323-FS20-T2-18
Deduction of Mechanisms
43 Microscopic Reversibility
a mechanism proposed once upon a time in square planar complexes
ligand substitution illustrative for microscopic reversibility
PtCl Cl
ClCl2-
PtCl Cl
ClCl 2-
PtCl Cl
ClClCl
PtCl Cl
ClCl
no mirror plane in the mechanism impedes the mr principle
+Cl- -Cl-
PtCl Cl
ClCl2-
PtCl Cl
ClClCl
PtCl Cl
ClCl 2-+Cl- -Cl-
instead
tbp transition state
06052020 CHE323-FS20-T2-19
Deduction of Mechanisms
44 Solvent effects
solvents
protic aprotic others
H-bondsweakly dipolar
H-bondshigh degreeof association
dipolar
stronglydipolarpolarizable
strongdipoles
dipoles e- pairdonors
aromaticspolar
aromaticapolar
apolar
try to find examples for this classification
06052020 CHE323-FS20-T2-20
Deduction of Mechanisms
44 Solvent effects
The solvent of a reaction may not only influence the rate
but also the overall mechanism path
in inorganic or organometallic reactions solvents often act as true ligands
solvents may stabilize destabilize ground ndash transition states or intermediates hellip
or they may be completely innocent
Fe FeOC
OC COCO
Fe FeOC
OC COCO
IFe
COCOI
+I2 +I- 2
Fe FeOC
OC COCO
Fe
COCOI
2
CHCl3
hexane
-+ -+ -+-
+
A B DC
06052020 CHE323-FS20-T2-21
Deduction of Mechanisms
44 Solvent effects
solvents may provide a solvent cage as the reaction vessel
the longer the reactants reside the faster the reaction
cage effect
contact ion-pair ion-pair separatedby one solvent layer
ion-pair separated by solvent layers
separated ion-pairs
z1 = z2 = 1 equiv 357 pmz1 = z2 = 2 equiv 1428 pmz1 = z2 = 3 equiv 3213 pm
critical distances for ion-pair formation
strongly dependent on dielectric constant ε
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
RTElnAlnk aminus= plot lnk vs 1T liefert A und Ea
ln(kT) vs T-1 yields ∆Hne (slope) and ∆Sne with ln(kBh) = 2376
26 kJmol bei RTand 132)458(logAS minus=∆ ne
06052020 CHE323-FS20-T2-2
Deduction of Mechanisms
42 Transition State Theory
Arrhenius
translates into TST kTln = kB
hln + ∆Sne
R∆Hne
RsdotT-
relation between Arrhenius and TST
the activation parameters ∆Hne and ∆Sne are very indicative for the mechanism
∆Hne = RmiddotTmiddotlnT
06052020 CHE323-FS20-T2-3
Deduction of Mechanisms
42 Transition State Theory
Example CoOC
OC PR3
PR3
CO
CO
COCo
OC
OC CO
PR3
CO
CO
2+
+ PR3
∆Hne = 37 kcalmol ∆Sne = + 25 eu
PRTVlnklnk 0
ne∆minus=
relates to volume of activation ∆Vne
thermodynamics dG = -SsdotdT + Vsdotdp kinetics
lnk depends linearly on the pressure
lnk (or logk) vs p gives a straight line with ∆Vne as slope
assuming that ∆Vne is pressure independent
what does this tell us with respect to mechanism
06052020 CHE323-FS20-T2-4
Deduction of Mechanisms
42 Transition State Theory
it gives an insight into the intimate mechanism of the rds
ground state transition states
D Id IaI A
∆Sne ++ + 0 - --∆Vne ++ + 0 - --
D and Id and A and Ia are often difficult to differentiate
∆Sne and ∆V ne give us a clear insight into the mechanism eg ligand exchange
06052020 CHE323-FS20-T2-5
Deduction of Mechanisms
42 Transition State Theory
interpretation of ∆Sne
interpretation of ∆Vne
larr associative
larr dissociative
∆Vdeg (H2O) = 18 cm3 mol
∆Vne = - 15 cm3 mol rArr A ndash process= + 15 cm3 mol rArr D ndash process
ligand self exchange
∆Vne represents the sum of the partial mol volumina in the transitions state
06052020 CHE323-FS20-T2-6
[M(OH2)6]n+
kex
[M(OH2)5(OH2)]n+ + H2O
bdquono reaction reaction ∆Go = O
for D processes k = kex
kex covers an extremely broad range
Deduction of Mechanisms
42 Transition State Theory
+ OH2
06052020 CHE323-FS20-T2-7
[M(OH2)6]n+Ka
[M(OH2)5(OH)] + H+
v = kex [M-OH] = kex middot Ka [M-OH2] [H]+
kex H+
[M(OH2)6]n+
∆H
(kJmiddotmol-1)∆S (eu) ∆V
(cm3sdotmol-1)Assigned mechanism
Ga3+ 67 +30 +5 Id
Ga(OH)2+ +6 Id
Ti3+ 43 -12 +1 Ia
Cr3+ 109 +12 -10 Ia
Cr(OH)2+ +3 I
Fe3+ 65 +12 -5 Ia
Fe(OH)2+ +7 Id
Ru3+ 90 -48 -8 Ia
Ru(OH)2+ +1 I
Rh3+ 131 +29 -4 Ia
Rh(OH)2+ +2 I
Ir3+ 131 +2 -6 Ia
Ir(OH)2+ +1 I
Deduction of Mechanisms
42 Transition State Theory
water self exchange rates indicative for substitution mechanisms in general
kex H2O
[M(OH2)6]n+
AH+ H+ + AKa A + B Pk2
][H][B][AHKkv a2 +
+
=
∆minus
∆sdot
∆minus
∆=
nene
RTHexp
RSexp
RTHexp
RSexp
hTkk
θa
θa22B
∆minus∆minus
∆+∆=
nene
RTHHexp
RSSexp
hTkk
θa2
θa2B
06052020 CHE323-FS20-T2-8
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
lets look at a pH dependent scheme
pm
the composed rate constant is k2middotKA
applying TST and vant Hoff
steady state System A + B Ik3
k-3I Pk4
k-3 gtgt k4 then [A][B]k
kkv3
43
minus
=
06052020 CHE323-FS20-T2-9
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
linearization we do not get ∆Hne but -∆Hne - ∆Hθ
we must know pre-equilibrium (Ka in this case) to get the true ∆Hne
Question find a situation for which ∆Gne is negative
eg the reaction becomes slower with increasing temperature
∆+∆minus∆minus
∆+∆minus∆=
neneminus
neneneminus
ne
RTHHHexp
RSSSexp
hTkk 433433B
06052020 CHE323-FS20-T2-10
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
we also get a linear profile the parameters are composed of the individual values
deviations from linearity in practice
a typical Eyring (or Arrhenius) plot
lnk
T
1T
slope yields ∆Hne
intercept yields ∆Sne
06052020 CHE323-FS20-T2-11
Deduction of Mechanisms
42 Transition State Theory
lnk
T
1T
lnk
T
1T
but sometimes the straight line looks rather like this
or like this hellip what does that mean
non-linearity often even in professional presentations still considered as linearhellip
06052020 CHE323-FS20-T2-12
A
P1
P2
k1
k2
)[A]k(kk[A]dt
d[A]21 +==minus
∆minus
∆+
∆minus
∆=
nenenene
RTHexp
RSexp
RTHexp
RSexp
hTkk 2211B
Deduction of Mechanisms
42 Transition State Theory
concurrent reaction schemes will result in non-linearity
what about a simple consecutive scheme like this one
A + B ABk1
k-1AB + C E + F
k2
06052020 CHE323-FS20-T2-13
the T profile of such a scheme is non-linear
Deduction of Mechanisms
42 Transition State Theory
it can however be dissected in two linear profiles
06052020 CHE323-FS20-T2-14
Deduction of Mechanisms
42 Transition State Theoryan simple example from practice
MH3N
H3N NH3
ONO
NH3
NH3
2+
MH3N
H3N NH3
NO2
NH3
NH3
2+
a linkage isomerization
activation parameters
Co Rh Ir
∆Sne -17+-3 -33+-7 -11+-4 eu
∆Hne 92 80 95 kJmol
∆Vne -67 -74 -59 cm3mol
is there an isokinetic relationship
what mechanism of isomerization do you suggest
The reaction mechanisms for the forward and the backward reaction
are identical they are mirror images of each other
A I I Pk1
k-1
k2
k-206052020 CHE323-FS20-T2-15
Deduction of Mechanisms
43 Microscopic Reversibility
This principle is always valid
even when multiple steps equilibria or branched reactions are involved
lets look at the simple reaction
if it were not valid we would run into problems with thermodynamics
[Fe(OH2)6]3+ + NCS- [Fe(NCS)(OH2)5]2+ + H2O
is pH dependent and can be described with
][FeNCS][H
kk]][NCS[Fe][H
kkdt
]d[FeNCS 221
321
2+
+minus
minusminus+
+
+
+minus
+=
0dt
]d[FeNCS2=
+
][Hkk
][Hkk
][NCS]O)[Fe(H]FeNCSO)[(HK
21
21
ee362
e2
52
+minus
minus
+
minus+
+
+
+==
06052020 CHE323-FS20-T2-16
Deduction of Mechanisms
43 Microscopic Reversibility
the rate law
forward reaction backward reaction
after equilibrium is achieved
and K becomes pH dependent
which it is obviously not according to reaction equation
e2
1ee3
1 ][FeNCSk][NCS][Fek +minus
minus+ =
e
e2
2e
ee3
2 ][H][FeNCSk
][H][NCS][Fek +
+
minus+
minus+
=
2
2
1
1
kk
kkK
minusminus
==
06052020 CHE323-FS20-T2-17
Deduction of Mechanisms
43 Microscopic Reversibility
Without consideration of the individual steps pH dependence
conflict with thermodynamics
the problem is solved by considering the microscopic reversibility of all
individual equilibria involved separately
pH independent path
pH dependent path
hence as it should be
06052020 CHE323-FS20-T2-18
Deduction of Mechanisms
43 Microscopic Reversibility
a mechanism proposed once upon a time in square planar complexes
ligand substitution illustrative for microscopic reversibility
PtCl Cl
ClCl2-
PtCl Cl
ClCl 2-
PtCl Cl
ClClCl
PtCl Cl
ClCl
no mirror plane in the mechanism impedes the mr principle
+Cl- -Cl-
PtCl Cl
ClCl2-
PtCl Cl
ClClCl
PtCl Cl
ClCl 2-+Cl- -Cl-
instead
tbp transition state
06052020 CHE323-FS20-T2-19
Deduction of Mechanisms
44 Solvent effects
solvents
protic aprotic others
H-bondsweakly dipolar
H-bondshigh degreeof association
dipolar
stronglydipolarpolarizable
strongdipoles
dipoles e- pairdonors
aromaticspolar
aromaticapolar
apolar
try to find examples for this classification
06052020 CHE323-FS20-T2-20
Deduction of Mechanisms
44 Solvent effects
The solvent of a reaction may not only influence the rate
but also the overall mechanism path
in inorganic or organometallic reactions solvents often act as true ligands
solvents may stabilize destabilize ground ndash transition states or intermediates hellip
or they may be completely innocent
Fe FeOC
OC COCO
Fe FeOC
OC COCO
IFe
COCOI
+I2 +I- 2
Fe FeOC
OC COCO
Fe
COCOI
2
CHCl3
hexane
-+ -+ -+-
+
A B DC
06052020 CHE323-FS20-T2-21
Deduction of Mechanisms
44 Solvent effects
solvents may provide a solvent cage as the reaction vessel
the longer the reactants reside the faster the reaction
cage effect
contact ion-pair ion-pair separatedby one solvent layer
ion-pair separated by solvent layers
separated ion-pairs
z1 = z2 = 1 equiv 357 pmz1 = z2 = 2 equiv 1428 pmz1 = z2 = 3 equiv 3213 pm
critical distances for ion-pair formation
strongly dependent on dielectric constant ε
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-3
Deduction of Mechanisms
42 Transition State Theory
Example CoOC
OC PR3
PR3
CO
CO
COCo
OC
OC CO
PR3
CO
CO
2+
+ PR3
∆Hne = 37 kcalmol ∆Sne = + 25 eu
PRTVlnklnk 0
ne∆minus=
relates to volume of activation ∆Vne
thermodynamics dG = -SsdotdT + Vsdotdp kinetics
lnk depends linearly on the pressure
lnk (or logk) vs p gives a straight line with ∆Vne as slope
assuming that ∆Vne is pressure independent
what does this tell us with respect to mechanism
06052020 CHE323-FS20-T2-4
Deduction of Mechanisms
42 Transition State Theory
it gives an insight into the intimate mechanism of the rds
ground state transition states
D Id IaI A
∆Sne ++ + 0 - --∆Vne ++ + 0 - --
D and Id and A and Ia are often difficult to differentiate
∆Sne and ∆V ne give us a clear insight into the mechanism eg ligand exchange
06052020 CHE323-FS20-T2-5
Deduction of Mechanisms
42 Transition State Theory
interpretation of ∆Sne
interpretation of ∆Vne
larr associative
larr dissociative
∆Vdeg (H2O) = 18 cm3 mol
∆Vne = - 15 cm3 mol rArr A ndash process= + 15 cm3 mol rArr D ndash process
ligand self exchange
∆Vne represents the sum of the partial mol volumina in the transitions state
06052020 CHE323-FS20-T2-6
[M(OH2)6]n+
kex
[M(OH2)5(OH2)]n+ + H2O
bdquono reaction reaction ∆Go = O
for D processes k = kex
kex covers an extremely broad range
Deduction of Mechanisms
42 Transition State Theory
+ OH2
06052020 CHE323-FS20-T2-7
[M(OH2)6]n+Ka
[M(OH2)5(OH)] + H+
v = kex [M-OH] = kex middot Ka [M-OH2] [H]+
kex H+
[M(OH2)6]n+
∆H
(kJmiddotmol-1)∆S (eu) ∆V
(cm3sdotmol-1)Assigned mechanism
Ga3+ 67 +30 +5 Id
Ga(OH)2+ +6 Id
Ti3+ 43 -12 +1 Ia
Cr3+ 109 +12 -10 Ia
Cr(OH)2+ +3 I
Fe3+ 65 +12 -5 Ia
Fe(OH)2+ +7 Id
Ru3+ 90 -48 -8 Ia
Ru(OH)2+ +1 I
Rh3+ 131 +29 -4 Ia
Rh(OH)2+ +2 I
Ir3+ 131 +2 -6 Ia
Ir(OH)2+ +1 I
Deduction of Mechanisms
42 Transition State Theory
water self exchange rates indicative for substitution mechanisms in general
kex H2O
[M(OH2)6]n+
AH+ H+ + AKa A + B Pk2
][H][B][AHKkv a2 +
+
=
∆minus
∆sdot
∆minus
∆=
nene
RTHexp
RSexp
RTHexp
RSexp
hTkk
θa
θa22B
∆minus∆minus
∆+∆=
nene
RTHHexp
RSSexp
hTkk
θa2
θa2B
06052020 CHE323-FS20-T2-8
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
lets look at a pH dependent scheme
pm
the composed rate constant is k2middotKA
applying TST and vant Hoff
steady state System A + B Ik3
k-3I Pk4
k-3 gtgt k4 then [A][B]k
kkv3
43
minus
=
06052020 CHE323-FS20-T2-9
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
linearization we do not get ∆Hne but -∆Hne - ∆Hθ
we must know pre-equilibrium (Ka in this case) to get the true ∆Hne
Question find a situation for which ∆Gne is negative
eg the reaction becomes slower with increasing temperature
∆+∆minus∆minus
∆+∆minus∆=
neneminus
neneneminus
ne
RTHHHexp
RSSSexp
hTkk 433433B
06052020 CHE323-FS20-T2-10
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
we also get a linear profile the parameters are composed of the individual values
deviations from linearity in practice
a typical Eyring (or Arrhenius) plot
lnk
T
1T
slope yields ∆Hne
intercept yields ∆Sne
06052020 CHE323-FS20-T2-11
Deduction of Mechanisms
42 Transition State Theory
lnk
T
1T
lnk
T
1T
but sometimes the straight line looks rather like this
or like this hellip what does that mean
non-linearity often even in professional presentations still considered as linearhellip
06052020 CHE323-FS20-T2-12
A
P1
P2
k1
k2
)[A]k(kk[A]dt
d[A]21 +==minus
∆minus
∆+
∆minus
∆=
nenenene
RTHexp
RSexp
RTHexp
RSexp
hTkk 2211B
Deduction of Mechanisms
42 Transition State Theory
concurrent reaction schemes will result in non-linearity
what about a simple consecutive scheme like this one
A + B ABk1
k-1AB + C E + F
k2
06052020 CHE323-FS20-T2-13
the T profile of such a scheme is non-linear
Deduction of Mechanisms
42 Transition State Theory
it can however be dissected in two linear profiles
06052020 CHE323-FS20-T2-14
Deduction of Mechanisms
42 Transition State Theoryan simple example from practice
MH3N
H3N NH3
ONO
NH3
NH3
2+
MH3N
H3N NH3
NO2
NH3
NH3
2+
a linkage isomerization
activation parameters
Co Rh Ir
∆Sne -17+-3 -33+-7 -11+-4 eu
∆Hne 92 80 95 kJmol
∆Vne -67 -74 -59 cm3mol
is there an isokinetic relationship
what mechanism of isomerization do you suggest
The reaction mechanisms for the forward and the backward reaction
are identical they are mirror images of each other
A I I Pk1
k-1
k2
k-206052020 CHE323-FS20-T2-15
Deduction of Mechanisms
43 Microscopic Reversibility
This principle is always valid
even when multiple steps equilibria or branched reactions are involved
lets look at the simple reaction
if it were not valid we would run into problems with thermodynamics
[Fe(OH2)6]3+ + NCS- [Fe(NCS)(OH2)5]2+ + H2O
is pH dependent and can be described with
][FeNCS][H
kk]][NCS[Fe][H
kkdt
]d[FeNCS 221
321
2+
+minus
minusminus+
+
+
+minus
+=
0dt
]d[FeNCS2=
+
][Hkk
][Hkk
][NCS]O)[Fe(H]FeNCSO)[(HK
21
21
ee362
e2
52
+minus
minus
+
minus+
+
+
+==
06052020 CHE323-FS20-T2-16
Deduction of Mechanisms
43 Microscopic Reversibility
the rate law
forward reaction backward reaction
after equilibrium is achieved
and K becomes pH dependent
which it is obviously not according to reaction equation
e2
1ee3
1 ][FeNCSk][NCS][Fek +minus
minus+ =
e
e2
2e
ee3
2 ][H][FeNCSk
][H][NCS][Fek +
+
minus+
minus+
=
2
2
1
1
kk
kkK
minusminus
==
06052020 CHE323-FS20-T2-17
Deduction of Mechanisms
43 Microscopic Reversibility
Without consideration of the individual steps pH dependence
conflict with thermodynamics
the problem is solved by considering the microscopic reversibility of all
individual equilibria involved separately
pH independent path
pH dependent path
hence as it should be
06052020 CHE323-FS20-T2-18
Deduction of Mechanisms
43 Microscopic Reversibility
a mechanism proposed once upon a time in square planar complexes
ligand substitution illustrative for microscopic reversibility
PtCl Cl
ClCl2-
PtCl Cl
ClCl 2-
PtCl Cl
ClClCl
PtCl Cl
ClCl
no mirror plane in the mechanism impedes the mr principle
+Cl- -Cl-
PtCl Cl
ClCl2-
PtCl Cl
ClClCl
PtCl Cl
ClCl 2-+Cl- -Cl-
instead
tbp transition state
06052020 CHE323-FS20-T2-19
Deduction of Mechanisms
44 Solvent effects
solvents
protic aprotic others
H-bondsweakly dipolar
H-bondshigh degreeof association
dipolar
stronglydipolarpolarizable
strongdipoles
dipoles e- pairdonors
aromaticspolar
aromaticapolar
apolar
try to find examples for this classification
06052020 CHE323-FS20-T2-20
Deduction of Mechanisms
44 Solvent effects
The solvent of a reaction may not only influence the rate
but also the overall mechanism path
in inorganic or organometallic reactions solvents often act as true ligands
solvents may stabilize destabilize ground ndash transition states or intermediates hellip
or they may be completely innocent
Fe FeOC
OC COCO
Fe FeOC
OC COCO
IFe
COCOI
+I2 +I- 2
Fe FeOC
OC COCO
Fe
COCOI
2
CHCl3
hexane
-+ -+ -+-
+
A B DC
06052020 CHE323-FS20-T2-21
Deduction of Mechanisms
44 Solvent effects
solvents may provide a solvent cage as the reaction vessel
the longer the reactants reside the faster the reaction
cage effect
contact ion-pair ion-pair separatedby one solvent layer
ion-pair separated by solvent layers
separated ion-pairs
z1 = z2 = 1 equiv 357 pmz1 = z2 = 2 equiv 1428 pmz1 = z2 = 3 equiv 3213 pm
critical distances for ion-pair formation
strongly dependent on dielectric constant ε
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-4
Deduction of Mechanisms
42 Transition State Theory
it gives an insight into the intimate mechanism of the rds
ground state transition states
D Id IaI A
∆Sne ++ + 0 - --∆Vne ++ + 0 - --
D and Id and A and Ia are often difficult to differentiate
∆Sne and ∆V ne give us a clear insight into the mechanism eg ligand exchange
06052020 CHE323-FS20-T2-5
Deduction of Mechanisms
42 Transition State Theory
interpretation of ∆Sne
interpretation of ∆Vne
larr associative
larr dissociative
∆Vdeg (H2O) = 18 cm3 mol
∆Vne = - 15 cm3 mol rArr A ndash process= + 15 cm3 mol rArr D ndash process
ligand self exchange
∆Vne represents the sum of the partial mol volumina in the transitions state
06052020 CHE323-FS20-T2-6
[M(OH2)6]n+
kex
[M(OH2)5(OH2)]n+ + H2O
bdquono reaction reaction ∆Go = O
for D processes k = kex
kex covers an extremely broad range
Deduction of Mechanisms
42 Transition State Theory
+ OH2
06052020 CHE323-FS20-T2-7
[M(OH2)6]n+Ka
[M(OH2)5(OH)] + H+
v = kex [M-OH] = kex middot Ka [M-OH2] [H]+
kex H+
[M(OH2)6]n+
∆H
(kJmiddotmol-1)∆S (eu) ∆V
(cm3sdotmol-1)Assigned mechanism
Ga3+ 67 +30 +5 Id
Ga(OH)2+ +6 Id
Ti3+ 43 -12 +1 Ia
Cr3+ 109 +12 -10 Ia
Cr(OH)2+ +3 I
Fe3+ 65 +12 -5 Ia
Fe(OH)2+ +7 Id
Ru3+ 90 -48 -8 Ia
Ru(OH)2+ +1 I
Rh3+ 131 +29 -4 Ia
Rh(OH)2+ +2 I
Ir3+ 131 +2 -6 Ia
Ir(OH)2+ +1 I
Deduction of Mechanisms
42 Transition State Theory
water self exchange rates indicative for substitution mechanisms in general
kex H2O
[M(OH2)6]n+
AH+ H+ + AKa A + B Pk2
][H][B][AHKkv a2 +
+
=
∆minus
∆sdot
∆minus
∆=
nene
RTHexp
RSexp
RTHexp
RSexp
hTkk
θa
θa22B
∆minus∆minus
∆+∆=
nene
RTHHexp
RSSexp
hTkk
θa2
θa2B
06052020 CHE323-FS20-T2-8
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
lets look at a pH dependent scheme
pm
the composed rate constant is k2middotKA
applying TST and vant Hoff
steady state System A + B Ik3
k-3I Pk4
k-3 gtgt k4 then [A][B]k
kkv3
43
minus
=
06052020 CHE323-FS20-T2-9
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
linearization we do not get ∆Hne but -∆Hne - ∆Hθ
we must know pre-equilibrium (Ka in this case) to get the true ∆Hne
Question find a situation for which ∆Gne is negative
eg the reaction becomes slower with increasing temperature
∆+∆minus∆minus
∆+∆minus∆=
neneminus
neneneminus
ne
RTHHHexp
RSSSexp
hTkk 433433B
06052020 CHE323-FS20-T2-10
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
we also get a linear profile the parameters are composed of the individual values
deviations from linearity in practice
a typical Eyring (or Arrhenius) plot
lnk
T
1T
slope yields ∆Hne
intercept yields ∆Sne
06052020 CHE323-FS20-T2-11
Deduction of Mechanisms
42 Transition State Theory
lnk
T
1T
lnk
T
1T
but sometimes the straight line looks rather like this
or like this hellip what does that mean
non-linearity often even in professional presentations still considered as linearhellip
06052020 CHE323-FS20-T2-12
A
P1
P2
k1
k2
)[A]k(kk[A]dt
d[A]21 +==minus
∆minus
∆+
∆minus
∆=
nenenene
RTHexp
RSexp
RTHexp
RSexp
hTkk 2211B
Deduction of Mechanisms
42 Transition State Theory
concurrent reaction schemes will result in non-linearity
what about a simple consecutive scheme like this one
A + B ABk1
k-1AB + C E + F
k2
06052020 CHE323-FS20-T2-13
the T profile of such a scheme is non-linear
Deduction of Mechanisms
42 Transition State Theory
it can however be dissected in two linear profiles
06052020 CHE323-FS20-T2-14
Deduction of Mechanisms
42 Transition State Theoryan simple example from practice
MH3N
H3N NH3
ONO
NH3
NH3
2+
MH3N
H3N NH3
NO2
NH3
NH3
2+
a linkage isomerization
activation parameters
Co Rh Ir
∆Sne -17+-3 -33+-7 -11+-4 eu
∆Hne 92 80 95 kJmol
∆Vne -67 -74 -59 cm3mol
is there an isokinetic relationship
what mechanism of isomerization do you suggest
The reaction mechanisms for the forward and the backward reaction
are identical they are mirror images of each other
A I I Pk1
k-1
k2
k-206052020 CHE323-FS20-T2-15
Deduction of Mechanisms
43 Microscopic Reversibility
This principle is always valid
even when multiple steps equilibria or branched reactions are involved
lets look at the simple reaction
if it were not valid we would run into problems with thermodynamics
[Fe(OH2)6]3+ + NCS- [Fe(NCS)(OH2)5]2+ + H2O
is pH dependent and can be described with
][FeNCS][H
kk]][NCS[Fe][H
kkdt
]d[FeNCS 221
321
2+
+minus
minusminus+
+
+
+minus
+=
0dt
]d[FeNCS2=
+
][Hkk
][Hkk
][NCS]O)[Fe(H]FeNCSO)[(HK
21
21
ee362
e2
52
+minus
minus
+
minus+
+
+
+==
06052020 CHE323-FS20-T2-16
Deduction of Mechanisms
43 Microscopic Reversibility
the rate law
forward reaction backward reaction
after equilibrium is achieved
and K becomes pH dependent
which it is obviously not according to reaction equation
e2
1ee3
1 ][FeNCSk][NCS][Fek +minus
minus+ =
e
e2
2e
ee3
2 ][H][FeNCSk
][H][NCS][Fek +
+
minus+
minus+
=
2
2
1
1
kk
kkK
minusminus
==
06052020 CHE323-FS20-T2-17
Deduction of Mechanisms
43 Microscopic Reversibility
Without consideration of the individual steps pH dependence
conflict with thermodynamics
the problem is solved by considering the microscopic reversibility of all
individual equilibria involved separately
pH independent path
pH dependent path
hence as it should be
06052020 CHE323-FS20-T2-18
Deduction of Mechanisms
43 Microscopic Reversibility
a mechanism proposed once upon a time in square planar complexes
ligand substitution illustrative for microscopic reversibility
PtCl Cl
ClCl2-
PtCl Cl
ClCl 2-
PtCl Cl
ClClCl
PtCl Cl
ClCl
no mirror plane in the mechanism impedes the mr principle
+Cl- -Cl-
PtCl Cl
ClCl2-
PtCl Cl
ClClCl
PtCl Cl
ClCl 2-+Cl- -Cl-
instead
tbp transition state
06052020 CHE323-FS20-T2-19
Deduction of Mechanisms
44 Solvent effects
solvents
protic aprotic others
H-bondsweakly dipolar
H-bondshigh degreeof association
dipolar
stronglydipolarpolarizable
strongdipoles
dipoles e- pairdonors
aromaticspolar
aromaticapolar
apolar
try to find examples for this classification
06052020 CHE323-FS20-T2-20
Deduction of Mechanisms
44 Solvent effects
The solvent of a reaction may not only influence the rate
but also the overall mechanism path
in inorganic or organometallic reactions solvents often act as true ligands
solvents may stabilize destabilize ground ndash transition states or intermediates hellip
or they may be completely innocent
Fe FeOC
OC COCO
Fe FeOC
OC COCO
IFe
COCOI
+I2 +I- 2
Fe FeOC
OC COCO
Fe
COCOI
2
CHCl3
hexane
-+ -+ -+-
+
A B DC
06052020 CHE323-FS20-T2-21
Deduction of Mechanisms
44 Solvent effects
solvents may provide a solvent cage as the reaction vessel
the longer the reactants reside the faster the reaction
cage effect
contact ion-pair ion-pair separatedby one solvent layer
ion-pair separated by solvent layers
separated ion-pairs
z1 = z2 = 1 equiv 357 pmz1 = z2 = 2 equiv 1428 pmz1 = z2 = 3 equiv 3213 pm
critical distances for ion-pair formation
strongly dependent on dielectric constant ε
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-5
Deduction of Mechanisms
42 Transition State Theory
interpretation of ∆Sne
interpretation of ∆Vne
larr associative
larr dissociative
∆Vdeg (H2O) = 18 cm3 mol
∆Vne = - 15 cm3 mol rArr A ndash process= + 15 cm3 mol rArr D ndash process
ligand self exchange
∆Vne represents the sum of the partial mol volumina in the transitions state
06052020 CHE323-FS20-T2-6
[M(OH2)6]n+
kex
[M(OH2)5(OH2)]n+ + H2O
bdquono reaction reaction ∆Go = O
for D processes k = kex
kex covers an extremely broad range
Deduction of Mechanisms
42 Transition State Theory
+ OH2
06052020 CHE323-FS20-T2-7
[M(OH2)6]n+Ka
[M(OH2)5(OH)] + H+
v = kex [M-OH] = kex middot Ka [M-OH2] [H]+
kex H+
[M(OH2)6]n+
∆H
(kJmiddotmol-1)∆S (eu) ∆V
(cm3sdotmol-1)Assigned mechanism
Ga3+ 67 +30 +5 Id
Ga(OH)2+ +6 Id
Ti3+ 43 -12 +1 Ia
Cr3+ 109 +12 -10 Ia
Cr(OH)2+ +3 I
Fe3+ 65 +12 -5 Ia
Fe(OH)2+ +7 Id
Ru3+ 90 -48 -8 Ia
Ru(OH)2+ +1 I
Rh3+ 131 +29 -4 Ia
Rh(OH)2+ +2 I
Ir3+ 131 +2 -6 Ia
Ir(OH)2+ +1 I
Deduction of Mechanisms
42 Transition State Theory
water self exchange rates indicative for substitution mechanisms in general
kex H2O
[M(OH2)6]n+
AH+ H+ + AKa A + B Pk2
][H][B][AHKkv a2 +
+
=
∆minus
∆sdot
∆minus
∆=
nene
RTHexp
RSexp
RTHexp
RSexp
hTkk
θa
θa22B
∆minus∆minus
∆+∆=
nene
RTHHexp
RSSexp
hTkk
θa2
θa2B
06052020 CHE323-FS20-T2-8
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
lets look at a pH dependent scheme
pm
the composed rate constant is k2middotKA
applying TST and vant Hoff
steady state System A + B Ik3
k-3I Pk4
k-3 gtgt k4 then [A][B]k
kkv3
43
minus
=
06052020 CHE323-FS20-T2-9
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
linearization we do not get ∆Hne but -∆Hne - ∆Hθ
we must know pre-equilibrium (Ka in this case) to get the true ∆Hne
Question find a situation for which ∆Gne is negative
eg the reaction becomes slower with increasing temperature
∆+∆minus∆minus
∆+∆minus∆=
neneminus
neneneminus
ne
RTHHHexp
RSSSexp
hTkk 433433B
06052020 CHE323-FS20-T2-10
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
we also get a linear profile the parameters are composed of the individual values
deviations from linearity in practice
a typical Eyring (or Arrhenius) plot
lnk
T
1T
slope yields ∆Hne
intercept yields ∆Sne
06052020 CHE323-FS20-T2-11
Deduction of Mechanisms
42 Transition State Theory
lnk
T
1T
lnk
T
1T
but sometimes the straight line looks rather like this
or like this hellip what does that mean
non-linearity often even in professional presentations still considered as linearhellip
06052020 CHE323-FS20-T2-12
A
P1
P2
k1
k2
)[A]k(kk[A]dt
d[A]21 +==minus
∆minus
∆+
∆minus
∆=
nenenene
RTHexp
RSexp
RTHexp
RSexp
hTkk 2211B
Deduction of Mechanisms
42 Transition State Theory
concurrent reaction schemes will result in non-linearity
what about a simple consecutive scheme like this one
A + B ABk1
k-1AB + C E + F
k2
06052020 CHE323-FS20-T2-13
the T profile of such a scheme is non-linear
Deduction of Mechanisms
42 Transition State Theory
it can however be dissected in two linear profiles
06052020 CHE323-FS20-T2-14
Deduction of Mechanisms
42 Transition State Theoryan simple example from practice
MH3N
H3N NH3
ONO
NH3
NH3
2+
MH3N
H3N NH3
NO2
NH3
NH3
2+
a linkage isomerization
activation parameters
Co Rh Ir
∆Sne -17+-3 -33+-7 -11+-4 eu
∆Hne 92 80 95 kJmol
∆Vne -67 -74 -59 cm3mol
is there an isokinetic relationship
what mechanism of isomerization do you suggest
The reaction mechanisms for the forward and the backward reaction
are identical they are mirror images of each other
A I I Pk1
k-1
k2
k-206052020 CHE323-FS20-T2-15
Deduction of Mechanisms
43 Microscopic Reversibility
This principle is always valid
even when multiple steps equilibria or branched reactions are involved
lets look at the simple reaction
if it were not valid we would run into problems with thermodynamics
[Fe(OH2)6]3+ + NCS- [Fe(NCS)(OH2)5]2+ + H2O
is pH dependent and can be described with
][FeNCS][H
kk]][NCS[Fe][H
kkdt
]d[FeNCS 221
321
2+
+minus
minusminus+
+
+
+minus
+=
0dt
]d[FeNCS2=
+
][Hkk
][Hkk
][NCS]O)[Fe(H]FeNCSO)[(HK
21
21
ee362
e2
52
+minus
minus
+
minus+
+
+
+==
06052020 CHE323-FS20-T2-16
Deduction of Mechanisms
43 Microscopic Reversibility
the rate law
forward reaction backward reaction
after equilibrium is achieved
and K becomes pH dependent
which it is obviously not according to reaction equation
e2
1ee3
1 ][FeNCSk][NCS][Fek +minus
minus+ =
e
e2
2e
ee3
2 ][H][FeNCSk
][H][NCS][Fek +
+
minus+
minus+
=
2
2
1
1
kk
kkK
minusminus
==
06052020 CHE323-FS20-T2-17
Deduction of Mechanisms
43 Microscopic Reversibility
Without consideration of the individual steps pH dependence
conflict with thermodynamics
the problem is solved by considering the microscopic reversibility of all
individual equilibria involved separately
pH independent path
pH dependent path
hence as it should be
06052020 CHE323-FS20-T2-18
Deduction of Mechanisms
43 Microscopic Reversibility
a mechanism proposed once upon a time in square planar complexes
ligand substitution illustrative for microscopic reversibility
PtCl Cl
ClCl2-
PtCl Cl
ClCl 2-
PtCl Cl
ClClCl
PtCl Cl
ClCl
no mirror plane in the mechanism impedes the mr principle
+Cl- -Cl-
PtCl Cl
ClCl2-
PtCl Cl
ClClCl
PtCl Cl
ClCl 2-+Cl- -Cl-
instead
tbp transition state
06052020 CHE323-FS20-T2-19
Deduction of Mechanisms
44 Solvent effects
solvents
protic aprotic others
H-bondsweakly dipolar
H-bondshigh degreeof association
dipolar
stronglydipolarpolarizable
strongdipoles
dipoles e- pairdonors
aromaticspolar
aromaticapolar
apolar
try to find examples for this classification
06052020 CHE323-FS20-T2-20
Deduction of Mechanisms
44 Solvent effects
The solvent of a reaction may not only influence the rate
but also the overall mechanism path
in inorganic or organometallic reactions solvents often act as true ligands
solvents may stabilize destabilize ground ndash transition states or intermediates hellip
or they may be completely innocent
Fe FeOC
OC COCO
Fe FeOC
OC COCO
IFe
COCOI
+I2 +I- 2
Fe FeOC
OC COCO
Fe
COCOI
2
CHCl3
hexane
-+ -+ -+-
+
A B DC
06052020 CHE323-FS20-T2-21
Deduction of Mechanisms
44 Solvent effects
solvents may provide a solvent cage as the reaction vessel
the longer the reactants reside the faster the reaction
cage effect
contact ion-pair ion-pair separatedby one solvent layer
ion-pair separated by solvent layers
separated ion-pairs
z1 = z2 = 1 equiv 357 pmz1 = z2 = 2 equiv 1428 pmz1 = z2 = 3 equiv 3213 pm
critical distances for ion-pair formation
strongly dependent on dielectric constant ε
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-6
[M(OH2)6]n+
kex
[M(OH2)5(OH2)]n+ + H2O
bdquono reaction reaction ∆Go = O
for D processes k = kex
kex covers an extremely broad range
Deduction of Mechanisms
42 Transition State Theory
+ OH2
06052020 CHE323-FS20-T2-7
[M(OH2)6]n+Ka
[M(OH2)5(OH)] + H+
v = kex [M-OH] = kex middot Ka [M-OH2] [H]+
kex H+
[M(OH2)6]n+
∆H
(kJmiddotmol-1)∆S (eu) ∆V
(cm3sdotmol-1)Assigned mechanism
Ga3+ 67 +30 +5 Id
Ga(OH)2+ +6 Id
Ti3+ 43 -12 +1 Ia
Cr3+ 109 +12 -10 Ia
Cr(OH)2+ +3 I
Fe3+ 65 +12 -5 Ia
Fe(OH)2+ +7 Id
Ru3+ 90 -48 -8 Ia
Ru(OH)2+ +1 I
Rh3+ 131 +29 -4 Ia
Rh(OH)2+ +2 I
Ir3+ 131 +2 -6 Ia
Ir(OH)2+ +1 I
Deduction of Mechanisms
42 Transition State Theory
water self exchange rates indicative for substitution mechanisms in general
kex H2O
[M(OH2)6]n+
AH+ H+ + AKa A + B Pk2
][H][B][AHKkv a2 +
+
=
∆minus
∆sdot
∆minus
∆=
nene
RTHexp
RSexp
RTHexp
RSexp
hTkk
θa
θa22B
∆minus∆minus
∆+∆=
nene
RTHHexp
RSSexp
hTkk
θa2
θa2B
06052020 CHE323-FS20-T2-8
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
lets look at a pH dependent scheme
pm
the composed rate constant is k2middotKA
applying TST and vant Hoff
steady state System A + B Ik3
k-3I Pk4
k-3 gtgt k4 then [A][B]k
kkv3
43
minus
=
06052020 CHE323-FS20-T2-9
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
linearization we do not get ∆Hne but -∆Hne - ∆Hθ
we must know pre-equilibrium (Ka in this case) to get the true ∆Hne
Question find a situation for which ∆Gne is negative
eg the reaction becomes slower with increasing temperature
∆+∆minus∆minus
∆+∆minus∆=
neneminus
neneneminus
ne
RTHHHexp
RSSSexp
hTkk 433433B
06052020 CHE323-FS20-T2-10
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
we also get a linear profile the parameters are composed of the individual values
deviations from linearity in practice
a typical Eyring (or Arrhenius) plot
lnk
T
1T
slope yields ∆Hne
intercept yields ∆Sne
06052020 CHE323-FS20-T2-11
Deduction of Mechanisms
42 Transition State Theory
lnk
T
1T
lnk
T
1T
but sometimes the straight line looks rather like this
or like this hellip what does that mean
non-linearity often even in professional presentations still considered as linearhellip
06052020 CHE323-FS20-T2-12
A
P1
P2
k1
k2
)[A]k(kk[A]dt
d[A]21 +==minus
∆minus
∆+
∆minus
∆=
nenenene
RTHexp
RSexp
RTHexp
RSexp
hTkk 2211B
Deduction of Mechanisms
42 Transition State Theory
concurrent reaction schemes will result in non-linearity
what about a simple consecutive scheme like this one
A + B ABk1
k-1AB + C E + F
k2
06052020 CHE323-FS20-T2-13
the T profile of such a scheme is non-linear
Deduction of Mechanisms
42 Transition State Theory
it can however be dissected in two linear profiles
06052020 CHE323-FS20-T2-14
Deduction of Mechanisms
42 Transition State Theoryan simple example from practice
MH3N
H3N NH3
ONO
NH3
NH3
2+
MH3N
H3N NH3
NO2
NH3
NH3
2+
a linkage isomerization
activation parameters
Co Rh Ir
∆Sne -17+-3 -33+-7 -11+-4 eu
∆Hne 92 80 95 kJmol
∆Vne -67 -74 -59 cm3mol
is there an isokinetic relationship
what mechanism of isomerization do you suggest
The reaction mechanisms for the forward and the backward reaction
are identical they are mirror images of each other
A I I Pk1
k-1
k2
k-206052020 CHE323-FS20-T2-15
Deduction of Mechanisms
43 Microscopic Reversibility
This principle is always valid
even when multiple steps equilibria or branched reactions are involved
lets look at the simple reaction
if it were not valid we would run into problems with thermodynamics
[Fe(OH2)6]3+ + NCS- [Fe(NCS)(OH2)5]2+ + H2O
is pH dependent and can be described with
][FeNCS][H
kk]][NCS[Fe][H
kkdt
]d[FeNCS 221
321
2+
+minus
minusminus+
+
+
+minus
+=
0dt
]d[FeNCS2=
+
][Hkk
][Hkk
][NCS]O)[Fe(H]FeNCSO)[(HK
21
21
ee362
e2
52
+minus
minus
+
minus+
+
+
+==
06052020 CHE323-FS20-T2-16
Deduction of Mechanisms
43 Microscopic Reversibility
the rate law
forward reaction backward reaction
after equilibrium is achieved
and K becomes pH dependent
which it is obviously not according to reaction equation
e2
1ee3
1 ][FeNCSk][NCS][Fek +minus
minus+ =
e
e2
2e
ee3
2 ][H][FeNCSk
][H][NCS][Fek +
+
minus+
minus+
=
2
2
1
1
kk
kkK
minusminus
==
06052020 CHE323-FS20-T2-17
Deduction of Mechanisms
43 Microscopic Reversibility
Without consideration of the individual steps pH dependence
conflict with thermodynamics
the problem is solved by considering the microscopic reversibility of all
individual equilibria involved separately
pH independent path
pH dependent path
hence as it should be
06052020 CHE323-FS20-T2-18
Deduction of Mechanisms
43 Microscopic Reversibility
a mechanism proposed once upon a time in square planar complexes
ligand substitution illustrative for microscopic reversibility
PtCl Cl
ClCl2-
PtCl Cl
ClCl 2-
PtCl Cl
ClClCl
PtCl Cl
ClCl
no mirror plane in the mechanism impedes the mr principle
+Cl- -Cl-
PtCl Cl
ClCl2-
PtCl Cl
ClClCl
PtCl Cl
ClCl 2-+Cl- -Cl-
instead
tbp transition state
06052020 CHE323-FS20-T2-19
Deduction of Mechanisms
44 Solvent effects
solvents
protic aprotic others
H-bondsweakly dipolar
H-bondshigh degreeof association
dipolar
stronglydipolarpolarizable
strongdipoles
dipoles e- pairdonors
aromaticspolar
aromaticapolar
apolar
try to find examples for this classification
06052020 CHE323-FS20-T2-20
Deduction of Mechanisms
44 Solvent effects
The solvent of a reaction may not only influence the rate
but also the overall mechanism path
in inorganic or organometallic reactions solvents often act as true ligands
solvents may stabilize destabilize ground ndash transition states or intermediates hellip
or they may be completely innocent
Fe FeOC
OC COCO
Fe FeOC
OC COCO
IFe
COCOI
+I2 +I- 2
Fe FeOC
OC COCO
Fe
COCOI
2
CHCl3
hexane
-+ -+ -+-
+
A B DC
06052020 CHE323-FS20-T2-21
Deduction of Mechanisms
44 Solvent effects
solvents may provide a solvent cage as the reaction vessel
the longer the reactants reside the faster the reaction
cage effect
contact ion-pair ion-pair separatedby one solvent layer
ion-pair separated by solvent layers
separated ion-pairs
z1 = z2 = 1 equiv 357 pmz1 = z2 = 2 equiv 1428 pmz1 = z2 = 3 equiv 3213 pm
critical distances for ion-pair formation
strongly dependent on dielectric constant ε
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-7
[M(OH2)6]n+Ka
[M(OH2)5(OH)] + H+
v = kex [M-OH] = kex middot Ka [M-OH2] [H]+
kex H+
[M(OH2)6]n+
∆H
(kJmiddotmol-1)∆S (eu) ∆V
(cm3sdotmol-1)Assigned mechanism
Ga3+ 67 +30 +5 Id
Ga(OH)2+ +6 Id
Ti3+ 43 -12 +1 Ia
Cr3+ 109 +12 -10 Ia
Cr(OH)2+ +3 I
Fe3+ 65 +12 -5 Ia
Fe(OH)2+ +7 Id
Ru3+ 90 -48 -8 Ia
Ru(OH)2+ +1 I
Rh3+ 131 +29 -4 Ia
Rh(OH)2+ +2 I
Ir3+ 131 +2 -6 Ia
Ir(OH)2+ +1 I
Deduction of Mechanisms
42 Transition State Theory
water self exchange rates indicative for substitution mechanisms in general
kex H2O
[M(OH2)6]n+
AH+ H+ + AKa A + B Pk2
][H][B][AHKkv a2 +
+
=
∆minus
∆sdot
∆minus
∆=
nene
RTHexp
RSexp
RTHexp
RSexp
hTkk
θa
θa22B
∆minus∆minus
∆+∆=
nene
RTHHexp
RSSexp
hTkk
θa2
θa2B
06052020 CHE323-FS20-T2-8
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
lets look at a pH dependent scheme
pm
the composed rate constant is k2middotKA
applying TST and vant Hoff
steady state System A + B Ik3
k-3I Pk4
k-3 gtgt k4 then [A][B]k
kkv3
43
minus
=
06052020 CHE323-FS20-T2-9
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
linearization we do not get ∆Hne but -∆Hne - ∆Hθ
we must know pre-equilibrium (Ka in this case) to get the true ∆Hne
Question find a situation for which ∆Gne is negative
eg the reaction becomes slower with increasing temperature
∆+∆minus∆minus
∆+∆minus∆=
neneminus
neneneminus
ne
RTHHHexp
RSSSexp
hTkk 433433B
06052020 CHE323-FS20-T2-10
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
we also get a linear profile the parameters are composed of the individual values
deviations from linearity in practice
a typical Eyring (or Arrhenius) plot
lnk
T
1T
slope yields ∆Hne
intercept yields ∆Sne
06052020 CHE323-FS20-T2-11
Deduction of Mechanisms
42 Transition State Theory
lnk
T
1T
lnk
T
1T
but sometimes the straight line looks rather like this
or like this hellip what does that mean
non-linearity often even in professional presentations still considered as linearhellip
06052020 CHE323-FS20-T2-12
A
P1
P2
k1
k2
)[A]k(kk[A]dt
d[A]21 +==minus
∆minus
∆+
∆minus
∆=
nenenene
RTHexp
RSexp
RTHexp
RSexp
hTkk 2211B
Deduction of Mechanisms
42 Transition State Theory
concurrent reaction schemes will result in non-linearity
what about a simple consecutive scheme like this one
A + B ABk1
k-1AB + C E + F
k2
06052020 CHE323-FS20-T2-13
the T profile of such a scheme is non-linear
Deduction of Mechanisms
42 Transition State Theory
it can however be dissected in two linear profiles
06052020 CHE323-FS20-T2-14
Deduction of Mechanisms
42 Transition State Theoryan simple example from practice
MH3N
H3N NH3
ONO
NH3
NH3
2+
MH3N
H3N NH3
NO2
NH3
NH3
2+
a linkage isomerization
activation parameters
Co Rh Ir
∆Sne -17+-3 -33+-7 -11+-4 eu
∆Hne 92 80 95 kJmol
∆Vne -67 -74 -59 cm3mol
is there an isokinetic relationship
what mechanism of isomerization do you suggest
The reaction mechanisms for the forward and the backward reaction
are identical they are mirror images of each other
A I I Pk1
k-1
k2
k-206052020 CHE323-FS20-T2-15
Deduction of Mechanisms
43 Microscopic Reversibility
This principle is always valid
even when multiple steps equilibria or branched reactions are involved
lets look at the simple reaction
if it were not valid we would run into problems with thermodynamics
[Fe(OH2)6]3+ + NCS- [Fe(NCS)(OH2)5]2+ + H2O
is pH dependent and can be described with
][FeNCS][H
kk]][NCS[Fe][H
kkdt
]d[FeNCS 221
321
2+
+minus
minusminus+
+
+
+minus
+=
0dt
]d[FeNCS2=
+
][Hkk
][Hkk
][NCS]O)[Fe(H]FeNCSO)[(HK
21
21
ee362
e2
52
+minus
minus
+
minus+
+
+
+==
06052020 CHE323-FS20-T2-16
Deduction of Mechanisms
43 Microscopic Reversibility
the rate law
forward reaction backward reaction
after equilibrium is achieved
and K becomes pH dependent
which it is obviously not according to reaction equation
e2
1ee3
1 ][FeNCSk][NCS][Fek +minus
minus+ =
e
e2
2e
ee3
2 ][H][FeNCSk
][H][NCS][Fek +
+
minus+
minus+
=
2
2
1
1
kk
kkK
minusminus
==
06052020 CHE323-FS20-T2-17
Deduction of Mechanisms
43 Microscopic Reversibility
Without consideration of the individual steps pH dependence
conflict with thermodynamics
the problem is solved by considering the microscopic reversibility of all
individual equilibria involved separately
pH independent path
pH dependent path
hence as it should be
06052020 CHE323-FS20-T2-18
Deduction of Mechanisms
43 Microscopic Reversibility
a mechanism proposed once upon a time in square planar complexes
ligand substitution illustrative for microscopic reversibility
PtCl Cl
ClCl2-
PtCl Cl
ClCl 2-
PtCl Cl
ClClCl
PtCl Cl
ClCl
no mirror plane in the mechanism impedes the mr principle
+Cl- -Cl-
PtCl Cl
ClCl2-
PtCl Cl
ClClCl
PtCl Cl
ClCl 2-+Cl- -Cl-
instead
tbp transition state
06052020 CHE323-FS20-T2-19
Deduction of Mechanisms
44 Solvent effects
solvents
protic aprotic others
H-bondsweakly dipolar
H-bondshigh degreeof association
dipolar
stronglydipolarpolarizable
strongdipoles
dipoles e- pairdonors
aromaticspolar
aromaticapolar
apolar
try to find examples for this classification
06052020 CHE323-FS20-T2-20
Deduction of Mechanisms
44 Solvent effects
The solvent of a reaction may not only influence the rate
but also the overall mechanism path
in inorganic or organometallic reactions solvents often act as true ligands
solvents may stabilize destabilize ground ndash transition states or intermediates hellip
or they may be completely innocent
Fe FeOC
OC COCO
Fe FeOC
OC COCO
IFe
COCOI
+I2 +I- 2
Fe FeOC
OC COCO
Fe
COCOI
2
CHCl3
hexane
-+ -+ -+-
+
A B DC
06052020 CHE323-FS20-T2-21
Deduction of Mechanisms
44 Solvent effects
solvents may provide a solvent cage as the reaction vessel
the longer the reactants reside the faster the reaction
cage effect
contact ion-pair ion-pair separatedby one solvent layer
ion-pair separated by solvent layers
separated ion-pairs
z1 = z2 = 1 equiv 357 pmz1 = z2 = 2 equiv 1428 pmz1 = z2 = 3 equiv 3213 pm
critical distances for ion-pair formation
strongly dependent on dielectric constant ε
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
AH+ H+ + AKa A + B Pk2
][H][B][AHKkv a2 +
+
=
∆minus
∆sdot
∆minus
∆=
nene
RTHexp
RSexp
RTHexp
RSexp
hTkk
θa
θa22B
∆minus∆minus
∆+∆=
nene
RTHHexp
RSSexp
hTkk
θa2
θa2B
06052020 CHE323-FS20-T2-8
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
lets look at a pH dependent scheme
pm
the composed rate constant is k2middotKA
applying TST and vant Hoff
steady state System A + B Ik3
k-3I Pk4
k-3 gtgt k4 then [A][B]k
kkv3
43
minus
=
06052020 CHE323-FS20-T2-9
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
linearization we do not get ∆Hne but -∆Hne - ∆Hθ
we must know pre-equilibrium (Ka in this case) to get the true ∆Hne
Question find a situation for which ∆Gne is negative
eg the reaction becomes slower with increasing temperature
∆+∆minus∆minus
∆+∆minus∆=
neneminus
neneneminus
ne
RTHHHexp
RSSSexp
hTkk 433433B
06052020 CHE323-FS20-T2-10
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
we also get a linear profile the parameters are composed of the individual values
deviations from linearity in practice
a typical Eyring (or Arrhenius) plot
lnk
T
1T
slope yields ∆Hne
intercept yields ∆Sne
06052020 CHE323-FS20-T2-11
Deduction of Mechanisms
42 Transition State Theory
lnk
T
1T
lnk
T
1T
but sometimes the straight line looks rather like this
or like this hellip what does that mean
non-linearity often even in professional presentations still considered as linearhellip
06052020 CHE323-FS20-T2-12
A
P1
P2
k1
k2
)[A]k(kk[A]dt
d[A]21 +==minus
∆minus
∆+
∆minus
∆=
nenenene
RTHexp
RSexp
RTHexp
RSexp
hTkk 2211B
Deduction of Mechanisms
42 Transition State Theory
concurrent reaction schemes will result in non-linearity
what about a simple consecutive scheme like this one
A + B ABk1
k-1AB + C E + F
k2
06052020 CHE323-FS20-T2-13
the T profile of such a scheme is non-linear
Deduction of Mechanisms
42 Transition State Theory
it can however be dissected in two linear profiles
06052020 CHE323-FS20-T2-14
Deduction of Mechanisms
42 Transition State Theoryan simple example from practice
MH3N
H3N NH3
ONO
NH3
NH3
2+
MH3N
H3N NH3
NO2
NH3
NH3
2+
a linkage isomerization
activation parameters
Co Rh Ir
∆Sne -17+-3 -33+-7 -11+-4 eu
∆Hne 92 80 95 kJmol
∆Vne -67 -74 -59 cm3mol
is there an isokinetic relationship
what mechanism of isomerization do you suggest
The reaction mechanisms for the forward and the backward reaction
are identical they are mirror images of each other
A I I Pk1
k-1
k2
k-206052020 CHE323-FS20-T2-15
Deduction of Mechanisms
43 Microscopic Reversibility
This principle is always valid
even when multiple steps equilibria or branched reactions are involved
lets look at the simple reaction
if it were not valid we would run into problems with thermodynamics
[Fe(OH2)6]3+ + NCS- [Fe(NCS)(OH2)5]2+ + H2O
is pH dependent and can be described with
][FeNCS][H
kk]][NCS[Fe][H
kkdt
]d[FeNCS 221
321
2+
+minus
minusminus+
+
+
+minus
+=
0dt
]d[FeNCS2=
+
][Hkk
][Hkk
][NCS]O)[Fe(H]FeNCSO)[(HK
21
21
ee362
e2
52
+minus
minus
+
minus+
+
+
+==
06052020 CHE323-FS20-T2-16
Deduction of Mechanisms
43 Microscopic Reversibility
the rate law
forward reaction backward reaction
after equilibrium is achieved
and K becomes pH dependent
which it is obviously not according to reaction equation
e2
1ee3
1 ][FeNCSk][NCS][Fek +minus
minus+ =
e
e2
2e
ee3
2 ][H][FeNCSk
][H][NCS][Fek +
+
minus+
minus+
=
2
2
1
1
kk
kkK
minusminus
==
06052020 CHE323-FS20-T2-17
Deduction of Mechanisms
43 Microscopic Reversibility
Without consideration of the individual steps pH dependence
conflict with thermodynamics
the problem is solved by considering the microscopic reversibility of all
individual equilibria involved separately
pH independent path
pH dependent path
hence as it should be
06052020 CHE323-FS20-T2-18
Deduction of Mechanisms
43 Microscopic Reversibility
a mechanism proposed once upon a time in square planar complexes
ligand substitution illustrative for microscopic reversibility
PtCl Cl
ClCl2-
PtCl Cl
ClCl 2-
PtCl Cl
ClClCl
PtCl Cl
ClCl
no mirror plane in the mechanism impedes the mr principle
+Cl- -Cl-
PtCl Cl
ClCl2-
PtCl Cl
ClClCl
PtCl Cl
ClCl 2-+Cl- -Cl-
instead
tbp transition state
06052020 CHE323-FS20-T2-19
Deduction of Mechanisms
44 Solvent effects
solvents
protic aprotic others
H-bondsweakly dipolar
H-bondshigh degreeof association
dipolar
stronglydipolarpolarizable
strongdipoles
dipoles e- pairdonors
aromaticspolar
aromaticapolar
apolar
try to find examples for this classification
06052020 CHE323-FS20-T2-20
Deduction of Mechanisms
44 Solvent effects
The solvent of a reaction may not only influence the rate
but also the overall mechanism path
in inorganic or organometallic reactions solvents often act as true ligands
solvents may stabilize destabilize ground ndash transition states or intermediates hellip
or they may be completely innocent
Fe FeOC
OC COCO
Fe FeOC
OC COCO
IFe
COCOI
+I2 +I- 2
Fe FeOC
OC COCO
Fe
COCOI
2
CHCl3
hexane
-+ -+ -+-
+
A B DC
06052020 CHE323-FS20-T2-21
Deduction of Mechanisms
44 Solvent effects
solvents may provide a solvent cage as the reaction vessel
the longer the reactants reside the faster the reaction
cage effect
contact ion-pair ion-pair separatedby one solvent layer
ion-pair separated by solvent layers
separated ion-pairs
z1 = z2 = 1 equiv 357 pmz1 = z2 = 2 equiv 1428 pmz1 = z2 = 3 equiv 3213 pm
critical distances for ion-pair formation
strongly dependent on dielectric constant ε
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
steady state System A + B Ik3
k-3I Pk4
k-3 gtgt k4 then [A][B]k
kkv3
43
minus
=
06052020 CHE323-FS20-T2-9
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
linearization we do not get ∆Hne but -∆Hne - ∆Hθ
we must know pre-equilibrium (Ka in this case) to get the true ∆Hne
Question find a situation for which ∆Gne is negative
eg the reaction becomes slower with increasing temperature
∆+∆minus∆minus
∆+∆minus∆=
neneminus
neneneminus
ne
RTHHHexp
RSSSexp
hTkk 433433B
06052020 CHE323-FS20-T2-10
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
we also get a linear profile the parameters are composed of the individual values
deviations from linearity in practice
a typical Eyring (or Arrhenius) plot
lnk
T
1T
slope yields ∆Hne
intercept yields ∆Sne
06052020 CHE323-FS20-T2-11
Deduction of Mechanisms
42 Transition State Theory
lnk
T
1T
lnk
T
1T
but sometimes the straight line looks rather like this
or like this hellip what does that mean
non-linearity often even in professional presentations still considered as linearhellip
06052020 CHE323-FS20-T2-12
A
P1
P2
k1
k2
)[A]k(kk[A]dt
d[A]21 +==minus
∆minus
∆+
∆minus
∆=
nenenene
RTHexp
RSexp
RTHexp
RSexp
hTkk 2211B
Deduction of Mechanisms
42 Transition State Theory
concurrent reaction schemes will result in non-linearity
what about a simple consecutive scheme like this one
A + B ABk1
k-1AB + C E + F
k2
06052020 CHE323-FS20-T2-13
the T profile of such a scheme is non-linear
Deduction of Mechanisms
42 Transition State Theory
it can however be dissected in two linear profiles
06052020 CHE323-FS20-T2-14
Deduction of Mechanisms
42 Transition State Theoryan simple example from practice
MH3N
H3N NH3
ONO
NH3
NH3
2+
MH3N
H3N NH3
NO2
NH3
NH3
2+
a linkage isomerization
activation parameters
Co Rh Ir
∆Sne -17+-3 -33+-7 -11+-4 eu
∆Hne 92 80 95 kJmol
∆Vne -67 -74 -59 cm3mol
is there an isokinetic relationship
what mechanism of isomerization do you suggest
The reaction mechanisms for the forward and the backward reaction
are identical they are mirror images of each other
A I I Pk1
k-1
k2
k-206052020 CHE323-FS20-T2-15
Deduction of Mechanisms
43 Microscopic Reversibility
This principle is always valid
even when multiple steps equilibria or branched reactions are involved
lets look at the simple reaction
if it were not valid we would run into problems with thermodynamics
[Fe(OH2)6]3+ + NCS- [Fe(NCS)(OH2)5]2+ + H2O
is pH dependent and can be described with
][FeNCS][H
kk]][NCS[Fe][H
kkdt
]d[FeNCS 221
321
2+
+minus
minusminus+
+
+
+minus
+=
0dt
]d[FeNCS2=
+
][Hkk
][Hkk
][NCS]O)[Fe(H]FeNCSO)[(HK
21
21
ee362
e2
52
+minus
minus
+
minus+
+
+
+==
06052020 CHE323-FS20-T2-16
Deduction of Mechanisms
43 Microscopic Reversibility
the rate law
forward reaction backward reaction
after equilibrium is achieved
and K becomes pH dependent
which it is obviously not according to reaction equation
e2
1ee3
1 ][FeNCSk][NCS][Fek +minus
minus+ =
e
e2
2e
ee3
2 ][H][FeNCSk
][H][NCS][Fek +
+
minus+
minus+
=
2
2
1
1
kk
kkK
minusminus
==
06052020 CHE323-FS20-T2-17
Deduction of Mechanisms
43 Microscopic Reversibility
Without consideration of the individual steps pH dependence
conflict with thermodynamics
the problem is solved by considering the microscopic reversibility of all
individual equilibria involved separately
pH independent path
pH dependent path
hence as it should be
06052020 CHE323-FS20-T2-18
Deduction of Mechanisms
43 Microscopic Reversibility
a mechanism proposed once upon a time in square planar complexes
ligand substitution illustrative for microscopic reversibility
PtCl Cl
ClCl2-
PtCl Cl
ClCl 2-
PtCl Cl
ClClCl
PtCl Cl
ClCl
no mirror plane in the mechanism impedes the mr principle
+Cl- -Cl-
PtCl Cl
ClCl2-
PtCl Cl
ClClCl
PtCl Cl
ClCl 2-+Cl- -Cl-
instead
tbp transition state
06052020 CHE323-FS20-T2-19
Deduction of Mechanisms
44 Solvent effects
solvents
protic aprotic others
H-bondsweakly dipolar
H-bondshigh degreeof association
dipolar
stronglydipolarpolarizable
strongdipoles
dipoles e- pairdonors
aromaticspolar
aromaticapolar
apolar
try to find examples for this classification
06052020 CHE323-FS20-T2-20
Deduction of Mechanisms
44 Solvent effects
The solvent of a reaction may not only influence the rate
but also the overall mechanism path
in inorganic or organometallic reactions solvents often act as true ligands
solvents may stabilize destabilize ground ndash transition states or intermediates hellip
or they may be completely innocent
Fe FeOC
OC COCO
Fe FeOC
OC COCO
IFe
COCOI
+I2 +I- 2
Fe FeOC
OC COCO
Fe
COCOI
2
CHCl3
hexane
-+ -+ -+-
+
A B DC
06052020 CHE323-FS20-T2-21
Deduction of Mechanisms
44 Solvent effects
solvents may provide a solvent cage as the reaction vessel
the longer the reactants reside the faster the reaction
cage effect
contact ion-pair ion-pair separatedby one solvent layer
ion-pair separated by solvent layers
separated ion-pairs
z1 = z2 = 1 equiv 357 pmz1 = z2 = 2 equiv 1428 pmz1 = z2 = 3 equiv 3213 pm
critical distances for ion-pair formation
strongly dependent on dielectric constant ε
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
∆+∆minus∆minus
∆+∆minus∆=
neneminus
neneneminus
ne
RTHHHexp
RSSSexp
hTkk 433433B
06052020 CHE323-FS20-T2-10
Deduction of Mechanisms
42 Transition State Theoryback to data interpretation most reactions are not one stephellip
we also get a linear profile the parameters are composed of the individual values
deviations from linearity in practice
a typical Eyring (or Arrhenius) plot
lnk
T
1T
slope yields ∆Hne
intercept yields ∆Sne
06052020 CHE323-FS20-T2-11
Deduction of Mechanisms
42 Transition State Theory
lnk
T
1T
lnk
T
1T
but sometimes the straight line looks rather like this
or like this hellip what does that mean
non-linearity often even in professional presentations still considered as linearhellip
06052020 CHE323-FS20-T2-12
A
P1
P2
k1
k2
)[A]k(kk[A]dt
d[A]21 +==minus
∆minus
∆+
∆minus
∆=
nenenene
RTHexp
RSexp
RTHexp
RSexp
hTkk 2211B
Deduction of Mechanisms
42 Transition State Theory
concurrent reaction schemes will result in non-linearity
what about a simple consecutive scheme like this one
A + B ABk1
k-1AB + C E + F
k2
06052020 CHE323-FS20-T2-13
the T profile of such a scheme is non-linear
Deduction of Mechanisms
42 Transition State Theory
it can however be dissected in two linear profiles
06052020 CHE323-FS20-T2-14
Deduction of Mechanisms
42 Transition State Theoryan simple example from practice
MH3N
H3N NH3
ONO
NH3
NH3
2+
MH3N
H3N NH3
NO2
NH3
NH3
2+
a linkage isomerization
activation parameters
Co Rh Ir
∆Sne -17+-3 -33+-7 -11+-4 eu
∆Hne 92 80 95 kJmol
∆Vne -67 -74 -59 cm3mol
is there an isokinetic relationship
what mechanism of isomerization do you suggest
The reaction mechanisms for the forward and the backward reaction
are identical they are mirror images of each other
A I I Pk1
k-1
k2
k-206052020 CHE323-FS20-T2-15
Deduction of Mechanisms
43 Microscopic Reversibility
This principle is always valid
even when multiple steps equilibria or branched reactions are involved
lets look at the simple reaction
if it were not valid we would run into problems with thermodynamics
[Fe(OH2)6]3+ + NCS- [Fe(NCS)(OH2)5]2+ + H2O
is pH dependent and can be described with
][FeNCS][H
kk]][NCS[Fe][H
kkdt
]d[FeNCS 221
321
2+
+minus
minusminus+
+
+
+minus
+=
0dt
]d[FeNCS2=
+
][Hkk
][Hkk
][NCS]O)[Fe(H]FeNCSO)[(HK
21
21
ee362
e2
52
+minus
minus
+
minus+
+
+
+==
06052020 CHE323-FS20-T2-16
Deduction of Mechanisms
43 Microscopic Reversibility
the rate law
forward reaction backward reaction
after equilibrium is achieved
and K becomes pH dependent
which it is obviously not according to reaction equation
e2
1ee3
1 ][FeNCSk][NCS][Fek +minus
minus+ =
e
e2
2e
ee3
2 ][H][FeNCSk
][H][NCS][Fek +
+
minus+
minus+
=
2
2
1
1
kk
kkK
minusminus
==
06052020 CHE323-FS20-T2-17
Deduction of Mechanisms
43 Microscopic Reversibility
Without consideration of the individual steps pH dependence
conflict with thermodynamics
the problem is solved by considering the microscopic reversibility of all
individual equilibria involved separately
pH independent path
pH dependent path
hence as it should be
06052020 CHE323-FS20-T2-18
Deduction of Mechanisms
43 Microscopic Reversibility
a mechanism proposed once upon a time in square planar complexes
ligand substitution illustrative for microscopic reversibility
PtCl Cl
ClCl2-
PtCl Cl
ClCl 2-
PtCl Cl
ClClCl
PtCl Cl
ClCl
no mirror plane in the mechanism impedes the mr principle
+Cl- -Cl-
PtCl Cl
ClCl2-
PtCl Cl
ClClCl
PtCl Cl
ClCl 2-+Cl- -Cl-
instead
tbp transition state
06052020 CHE323-FS20-T2-19
Deduction of Mechanisms
44 Solvent effects
solvents
protic aprotic others
H-bondsweakly dipolar
H-bondshigh degreeof association
dipolar
stronglydipolarpolarizable
strongdipoles
dipoles e- pairdonors
aromaticspolar
aromaticapolar
apolar
try to find examples for this classification
06052020 CHE323-FS20-T2-20
Deduction of Mechanisms
44 Solvent effects
The solvent of a reaction may not only influence the rate
but also the overall mechanism path
in inorganic or organometallic reactions solvents often act as true ligands
solvents may stabilize destabilize ground ndash transition states or intermediates hellip
or they may be completely innocent
Fe FeOC
OC COCO
Fe FeOC
OC COCO
IFe
COCOI
+I2 +I- 2
Fe FeOC
OC COCO
Fe
COCOI
2
CHCl3
hexane
-+ -+ -+-
+
A B DC
06052020 CHE323-FS20-T2-21
Deduction of Mechanisms
44 Solvent effects
solvents may provide a solvent cage as the reaction vessel
the longer the reactants reside the faster the reaction
cage effect
contact ion-pair ion-pair separatedby one solvent layer
ion-pair separated by solvent layers
separated ion-pairs
z1 = z2 = 1 equiv 357 pmz1 = z2 = 2 equiv 1428 pmz1 = z2 = 3 equiv 3213 pm
critical distances for ion-pair formation
strongly dependent on dielectric constant ε
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-11
Deduction of Mechanisms
42 Transition State Theory
lnk
T
1T
lnk
T
1T
but sometimes the straight line looks rather like this
or like this hellip what does that mean
non-linearity often even in professional presentations still considered as linearhellip
06052020 CHE323-FS20-T2-12
A
P1
P2
k1
k2
)[A]k(kk[A]dt
d[A]21 +==minus
∆minus
∆+
∆minus
∆=
nenenene
RTHexp
RSexp
RTHexp
RSexp
hTkk 2211B
Deduction of Mechanisms
42 Transition State Theory
concurrent reaction schemes will result in non-linearity
what about a simple consecutive scheme like this one
A + B ABk1
k-1AB + C E + F
k2
06052020 CHE323-FS20-T2-13
the T profile of such a scheme is non-linear
Deduction of Mechanisms
42 Transition State Theory
it can however be dissected in two linear profiles
06052020 CHE323-FS20-T2-14
Deduction of Mechanisms
42 Transition State Theoryan simple example from practice
MH3N
H3N NH3
ONO
NH3
NH3
2+
MH3N
H3N NH3
NO2
NH3
NH3
2+
a linkage isomerization
activation parameters
Co Rh Ir
∆Sne -17+-3 -33+-7 -11+-4 eu
∆Hne 92 80 95 kJmol
∆Vne -67 -74 -59 cm3mol
is there an isokinetic relationship
what mechanism of isomerization do you suggest
The reaction mechanisms for the forward and the backward reaction
are identical they are mirror images of each other
A I I Pk1
k-1
k2
k-206052020 CHE323-FS20-T2-15
Deduction of Mechanisms
43 Microscopic Reversibility
This principle is always valid
even when multiple steps equilibria or branched reactions are involved
lets look at the simple reaction
if it were not valid we would run into problems with thermodynamics
[Fe(OH2)6]3+ + NCS- [Fe(NCS)(OH2)5]2+ + H2O
is pH dependent and can be described with
][FeNCS][H
kk]][NCS[Fe][H
kkdt
]d[FeNCS 221
321
2+
+minus
minusminus+
+
+
+minus
+=
0dt
]d[FeNCS2=
+
][Hkk
][Hkk
][NCS]O)[Fe(H]FeNCSO)[(HK
21
21
ee362
e2
52
+minus
minus
+
minus+
+
+
+==
06052020 CHE323-FS20-T2-16
Deduction of Mechanisms
43 Microscopic Reversibility
the rate law
forward reaction backward reaction
after equilibrium is achieved
and K becomes pH dependent
which it is obviously not according to reaction equation
e2
1ee3
1 ][FeNCSk][NCS][Fek +minus
minus+ =
e
e2
2e
ee3
2 ][H][FeNCSk
][H][NCS][Fek +
+
minus+
minus+
=
2
2
1
1
kk
kkK
minusminus
==
06052020 CHE323-FS20-T2-17
Deduction of Mechanisms
43 Microscopic Reversibility
Without consideration of the individual steps pH dependence
conflict with thermodynamics
the problem is solved by considering the microscopic reversibility of all
individual equilibria involved separately
pH independent path
pH dependent path
hence as it should be
06052020 CHE323-FS20-T2-18
Deduction of Mechanisms
43 Microscopic Reversibility
a mechanism proposed once upon a time in square planar complexes
ligand substitution illustrative for microscopic reversibility
PtCl Cl
ClCl2-
PtCl Cl
ClCl 2-
PtCl Cl
ClClCl
PtCl Cl
ClCl
no mirror plane in the mechanism impedes the mr principle
+Cl- -Cl-
PtCl Cl
ClCl2-
PtCl Cl
ClClCl
PtCl Cl
ClCl 2-+Cl- -Cl-
instead
tbp transition state
06052020 CHE323-FS20-T2-19
Deduction of Mechanisms
44 Solvent effects
solvents
protic aprotic others
H-bondsweakly dipolar
H-bondshigh degreeof association
dipolar
stronglydipolarpolarizable
strongdipoles
dipoles e- pairdonors
aromaticspolar
aromaticapolar
apolar
try to find examples for this classification
06052020 CHE323-FS20-T2-20
Deduction of Mechanisms
44 Solvent effects
The solvent of a reaction may not only influence the rate
but also the overall mechanism path
in inorganic or organometallic reactions solvents often act as true ligands
solvents may stabilize destabilize ground ndash transition states or intermediates hellip
or they may be completely innocent
Fe FeOC
OC COCO
Fe FeOC
OC COCO
IFe
COCOI
+I2 +I- 2
Fe FeOC
OC COCO
Fe
COCOI
2
CHCl3
hexane
-+ -+ -+-
+
A B DC
06052020 CHE323-FS20-T2-21
Deduction of Mechanisms
44 Solvent effects
solvents may provide a solvent cage as the reaction vessel
the longer the reactants reside the faster the reaction
cage effect
contact ion-pair ion-pair separatedby one solvent layer
ion-pair separated by solvent layers
separated ion-pairs
z1 = z2 = 1 equiv 357 pmz1 = z2 = 2 equiv 1428 pmz1 = z2 = 3 equiv 3213 pm
critical distances for ion-pair formation
strongly dependent on dielectric constant ε
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-12
A
P1
P2
k1
k2
)[A]k(kk[A]dt
d[A]21 +==minus
∆minus
∆+
∆minus
∆=
nenenene
RTHexp
RSexp
RTHexp
RSexp
hTkk 2211B
Deduction of Mechanisms
42 Transition State Theory
concurrent reaction schemes will result in non-linearity
what about a simple consecutive scheme like this one
A + B ABk1
k-1AB + C E + F
k2
06052020 CHE323-FS20-T2-13
the T profile of such a scheme is non-linear
Deduction of Mechanisms
42 Transition State Theory
it can however be dissected in two linear profiles
06052020 CHE323-FS20-T2-14
Deduction of Mechanisms
42 Transition State Theoryan simple example from practice
MH3N
H3N NH3
ONO
NH3
NH3
2+
MH3N
H3N NH3
NO2
NH3
NH3
2+
a linkage isomerization
activation parameters
Co Rh Ir
∆Sne -17+-3 -33+-7 -11+-4 eu
∆Hne 92 80 95 kJmol
∆Vne -67 -74 -59 cm3mol
is there an isokinetic relationship
what mechanism of isomerization do you suggest
The reaction mechanisms for the forward and the backward reaction
are identical they are mirror images of each other
A I I Pk1
k-1
k2
k-206052020 CHE323-FS20-T2-15
Deduction of Mechanisms
43 Microscopic Reversibility
This principle is always valid
even when multiple steps equilibria or branched reactions are involved
lets look at the simple reaction
if it were not valid we would run into problems with thermodynamics
[Fe(OH2)6]3+ + NCS- [Fe(NCS)(OH2)5]2+ + H2O
is pH dependent and can be described with
][FeNCS][H
kk]][NCS[Fe][H
kkdt
]d[FeNCS 221
321
2+
+minus
minusminus+
+
+
+minus
+=
0dt
]d[FeNCS2=
+
][Hkk
][Hkk
][NCS]O)[Fe(H]FeNCSO)[(HK
21
21
ee362
e2
52
+minus
minus
+
minus+
+
+
+==
06052020 CHE323-FS20-T2-16
Deduction of Mechanisms
43 Microscopic Reversibility
the rate law
forward reaction backward reaction
after equilibrium is achieved
and K becomes pH dependent
which it is obviously not according to reaction equation
e2
1ee3
1 ][FeNCSk][NCS][Fek +minus
minus+ =
e
e2
2e
ee3
2 ][H][FeNCSk
][H][NCS][Fek +
+
minus+
minus+
=
2
2
1
1
kk
kkK
minusminus
==
06052020 CHE323-FS20-T2-17
Deduction of Mechanisms
43 Microscopic Reversibility
Without consideration of the individual steps pH dependence
conflict with thermodynamics
the problem is solved by considering the microscopic reversibility of all
individual equilibria involved separately
pH independent path
pH dependent path
hence as it should be
06052020 CHE323-FS20-T2-18
Deduction of Mechanisms
43 Microscopic Reversibility
a mechanism proposed once upon a time in square planar complexes
ligand substitution illustrative for microscopic reversibility
PtCl Cl
ClCl2-
PtCl Cl
ClCl 2-
PtCl Cl
ClClCl
PtCl Cl
ClCl
no mirror plane in the mechanism impedes the mr principle
+Cl- -Cl-
PtCl Cl
ClCl2-
PtCl Cl
ClClCl
PtCl Cl
ClCl 2-+Cl- -Cl-
instead
tbp transition state
06052020 CHE323-FS20-T2-19
Deduction of Mechanisms
44 Solvent effects
solvents
protic aprotic others
H-bondsweakly dipolar
H-bondshigh degreeof association
dipolar
stronglydipolarpolarizable
strongdipoles
dipoles e- pairdonors
aromaticspolar
aromaticapolar
apolar
try to find examples for this classification
06052020 CHE323-FS20-T2-20
Deduction of Mechanisms
44 Solvent effects
The solvent of a reaction may not only influence the rate
but also the overall mechanism path
in inorganic or organometallic reactions solvents often act as true ligands
solvents may stabilize destabilize ground ndash transition states or intermediates hellip
or they may be completely innocent
Fe FeOC
OC COCO
Fe FeOC
OC COCO
IFe
COCOI
+I2 +I- 2
Fe FeOC
OC COCO
Fe
COCOI
2
CHCl3
hexane
-+ -+ -+-
+
A B DC
06052020 CHE323-FS20-T2-21
Deduction of Mechanisms
44 Solvent effects
solvents may provide a solvent cage as the reaction vessel
the longer the reactants reside the faster the reaction
cage effect
contact ion-pair ion-pair separatedby one solvent layer
ion-pair separated by solvent layers
separated ion-pairs
z1 = z2 = 1 equiv 357 pmz1 = z2 = 2 equiv 1428 pmz1 = z2 = 3 equiv 3213 pm
critical distances for ion-pair formation
strongly dependent on dielectric constant ε
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-13
the T profile of such a scheme is non-linear
Deduction of Mechanisms
42 Transition State Theory
it can however be dissected in two linear profiles
06052020 CHE323-FS20-T2-14
Deduction of Mechanisms
42 Transition State Theoryan simple example from practice
MH3N
H3N NH3
ONO
NH3
NH3
2+
MH3N
H3N NH3
NO2
NH3
NH3
2+
a linkage isomerization
activation parameters
Co Rh Ir
∆Sne -17+-3 -33+-7 -11+-4 eu
∆Hne 92 80 95 kJmol
∆Vne -67 -74 -59 cm3mol
is there an isokinetic relationship
what mechanism of isomerization do you suggest
The reaction mechanisms for the forward and the backward reaction
are identical they are mirror images of each other
A I I Pk1
k-1
k2
k-206052020 CHE323-FS20-T2-15
Deduction of Mechanisms
43 Microscopic Reversibility
This principle is always valid
even when multiple steps equilibria or branched reactions are involved
lets look at the simple reaction
if it were not valid we would run into problems with thermodynamics
[Fe(OH2)6]3+ + NCS- [Fe(NCS)(OH2)5]2+ + H2O
is pH dependent and can be described with
][FeNCS][H
kk]][NCS[Fe][H
kkdt
]d[FeNCS 221
321
2+
+minus
minusminus+
+
+
+minus
+=
0dt
]d[FeNCS2=
+
][Hkk
][Hkk
][NCS]O)[Fe(H]FeNCSO)[(HK
21
21
ee362
e2
52
+minus
minus
+
minus+
+
+
+==
06052020 CHE323-FS20-T2-16
Deduction of Mechanisms
43 Microscopic Reversibility
the rate law
forward reaction backward reaction
after equilibrium is achieved
and K becomes pH dependent
which it is obviously not according to reaction equation
e2
1ee3
1 ][FeNCSk][NCS][Fek +minus
minus+ =
e
e2
2e
ee3
2 ][H][FeNCSk
][H][NCS][Fek +
+
minus+
minus+
=
2
2
1
1
kk
kkK
minusminus
==
06052020 CHE323-FS20-T2-17
Deduction of Mechanisms
43 Microscopic Reversibility
Without consideration of the individual steps pH dependence
conflict with thermodynamics
the problem is solved by considering the microscopic reversibility of all
individual equilibria involved separately
pH independent path
pH dependent path
hence as it should be
06052020 CHE323-FS20-T2-18
Deduction of Mechanisms
43 Microscopic Reversibility
a mechanism proposed once upon a time in square planar complexes
ligand substitution illustrative for microscopic reversibility
PtCl Cl
ClCl2-
PtCl Cl
ClCl 2-
PtCl Cl
ClClCl
PtCl Cl
ClCl
no mirror plane in the mechanism impedes the mr principle
+Cl- -Cl-
PtCl Cl
ClCl2-
PtCl Cl
ClClCl
PtCl Cl
ClCl 2-+Cl- -Cl-
instead
tbp transition state
06052020 CHE323-FS20-T2-19
Deduction of Mechanisms
44 Solvent effects
solvents
protic aprotic others
H-bondsweakly dipolar
H-bondshigh degreeof association
dipolar
stronglydipolarpolarizable
strongdipoles
dipoles e- pairdonors
aromaticspolar
aromaticapolar
apolar
try to find examples for this classification
06052020 CHE323-FS20-T2-20
Deduction of Mechanisms
44 Solvent effects
The solvent of a reaction may not only influence the rate
but also the overall mechanism path
in inorganic or organometallic reactions solvents often act as true ligands
solvents may stabilize destabilize ground ndash transition states or intermediates hellip
or they may be completely innocent
Fe FeOC
OC COCO
Fe FeOC
OC COCO
IFe
COCOI
+I2 +I- 2
Fe FeOC
OC COCO
Fe
COCOI
2
CHCl3
hexane
-+ -+ -+-
+
A B DC
06052020 CHE323-FS20-T2-21
Deduction of Mechanisms
44 Solvent effects
solvents may provide a solvent cage as the reaction vessel
the longer the reactants reside the faster the reaction
cage effect
contact ion-pair ion-pair separatedby one solvent layer
ion-pair separated by solvent layers
separated ion-pairs
z1 = z2 = 1 equiv 357 pmz1 = z2 = 2 equiv 1428 pmz1 = z2 = 3 equiv 3213 pm
critical distances for ion-pair formation
strongly dependent on dielectric constant ε
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-14
Deduction of Mechanisms
42 Transition State Theoryan simple example from practice
MH3N
H3N NH3
ONO
NH3
NH3
2+
MH3N
H3N NH3
NO2
NH3
NH3
2+
a linkage isomerization
activation parameters
Co Rh Ir
∆Sne -17+-3 -33+-7 -11+-4 eu
∆Hne 92 80 95 kJmol
∆Vne -67 -74 -59 cm3mol
is there an isokinetic relationship
what mechanism of isomerization do you suggest
The reaction mechanisms for the forward and the backward reaction
are identical they are mirror images of each other
A I I Pk1
k-1
k2
k-206052020 CHE323-FS20-T2-15
Deduction of Mechanisms
43 Microscopic Reversibility
This principle is always valid
even when multiple steps equilibria or branched reactions are involved
lets look at the simple reaction
if it were not valid we would run into problems with thermodynamics
[Fe(OH2)6]3+ + NCS- [Fe(NCS)(OH2)5]2+ + H2O
is pH dependent and can be described with
][FeNCS][H
kk]][NCS[Fe][H
kkdt
]d[FeNCS 221
321
2+
+minus
minusminus+
+
+
+minus
+=
0dt
]d[FeNCS2=
+
][Hkk
][Hkk
][NCS]O)[Fe(H]FeNCSO)[(HK
21
21
ee362
e2
52
+minus
minus
+
minus+
+
+
+==
06052020 CHE323-FS20-T2-16
Deduction of Mechanisms
43 Microscopic Reversibility
the rate law
forward reaction backward reaction
after equilibrium is achieved
and K becomes pH dependent
which it is obviously not according to reaction equation
e2
1ee3
1 ][FeNCSk][NCS][Fek +minus
minus+ =
e
e2
2e
ee3
2 ][H][FeNCSk
][H][NCS][Fek +
+
minus+
minus+
=
2
2
1
1
kk
kkK
minusminus
==
06052020 CHE323-FS20-T2-17
Deduction of Mechanisms
43 Microscopic Reversibility
Without consideration of the individual steps pH dependence
conflict with thermodynamics
the problem is solved by considering the microscopic reversibility of all
individual equilibria involved separately
pH independent path
pH dependent path
hence as it should be
06052020 CHE323-FS20-T2-18
Deduction of Mechanisms
43 Microscopic Reversibility
a mechanism proposed once upon a time in square planar complexes
ligand substitution illustrative for microscopic reversibility
PtCl Cl
ClCl2-
PtCl Cl
ClCl 2-
PtCl Cl
ClClCl
PtCl Cl
ClCl
no mirror plane in the mechanism impedes the mr principle
+Cl- -Cl-
PtCl Cl
ClCl2-
PtCl Cl
ClClCl
PtCl Cl
ClCl 2-+Cl- -Cl-
instead
tbp transition state
06052020 CHE323-FS20-T2-19
Deduction of Mechanisms
44 Solvent effects
solvents
protic aprotic others
H-bondsweakly dipolar
H-bondshigh degreeof association
dipolar
stronglydipolarpolarizable
strongdipoles
dipoles e- pairdonors
aromaticspolar
aromaticapolar
apolar
try to find examples for this classification
06052020 CHE323-FS20-T2-20
Deduction of Mechanisms
44 Solvent effects
The solvent of a reaction may not only influence the rate
but also the overall mechanism path
in inorganic or organometallic reactions solvents often act as true ligands
solvents may stabilize destabilize ground ndash transition states or intermediates hellip
or they may be completely innocent
Fe FeOC
OC COCO
Fe FeOC
OC COCO
IFe
COCOI
+I2 +I- 2
Fe FeOC
OC COCO
Fe
COCOI
2
CHCl3
hexane
-+ -+ -+-
+
A B DC
06052020 CHE323-FS20-T2-21
Deduction of Mechanisms
44 Solvent effects
solvents may provide a solvent cage as the reaction vessel
the longer the reactants reside the faster the reaction
cage effect
contact ion-pair ion-pair separatedby one solvent layer
ion-pair separated by solvent layers
separated ion-pairs
z1 = z2 = 1 equiv 357 pmz1 = z2 = 2 equiv 1428 pmz1 = z2 = 3 equiv 3213 pm
critical distances for ion-pair formation
strongly dependent on dielectric constant ε
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
The reaction mechanisms for the forward and the backward reaction
are identical they are mirror images of each other
A I I Pk1
k-1
k2
k-206052020 CHE323-FS20-T2-15
Deduction of Mechanisms
43 Microscopic Reversibility
This principle is always valid
even when multiple steps equilibria or branched reactions are involved
lets look at the simple reaction
if it were not valid we would run into problems with thermodynamics
[Fe(OH2)6]3+ + NCS- [Fe(NCS)(OH2)5]2+ + H2O
is pH dependent and can be described with
][FeNCS][H
kk]][NCS[Fe][H
kkdt
]d[FeNCS 221
321
2+
+minus
minusminus+
+
+
+minus
+=
0dt
]d[FeNCS2=
+
][Hkk
][Hkk
][NCS]O)[Fe(H]FeNCSO)[(HK
21
21
ee362
e2
52
+minus
minus
+
minus+
+
+
+==
06052020 CHE323-FS20-T2-16
Deduction of Mechanisms
43 Microscopic Reversibility
the rate law
forward reaction backward reaction
after equilibrium is achieved
and K becomes pH dependent
which it is obviously not according to reaction equation
e2
1ee3
1 ][FeNCSk][NCS][Fek +minus
minus+ =
e
e2
2e
ee3
2 ][H][FeNCSk
][H][NCS][Fek +
+
minus+
minus+
=
2
2
1
1
kk
kkK
minusminus
==
06052020 CHE323-FS20-T2-17
Deduction of Mechanisms
43 Microscopic Reversibility
Without consideration of the individual steps pH dependence
conflict with thermodynamics
the problem is solved by considering the microscopic reversibility of all
individual equilibria involved separately
pH independent path
pH dependent path
hence as it should be
06052020 CHE323-FS20-T2-18
Deduction of Mechanisms
43 Microscopic Reversibility
a mechanism proposed once upon a time in square planar complexes
ligand substitution illustrative for microscopic reversibility
PtCl Cl
ClCl2-
PtCl Cl
ClCl 2-
PtCl Cl
ClClCl
PtCl Cl
ClCl
no mirror plane in the mechanism impedes the mr principle
+Cl- -Cl-
PtCl Cl
ClCl2-
PtCl Cl
ClClCl
PtCl Cl
ClCl 2-+Cl- -Cl-
instead
tbp transition state
06052020 CHE323-FS20-T2-19
Deduction of Mechanisms
44 Solvent effects
solvents
protic aprotic others
H-bondsweakly dipolar
H-bondshigh degreeof association
dipolar
stronglydipolarpolarizable
strongdipoles
dipoles e- pairdonors
aromaticspolar
aromaticapolar
apolar
try to find examples for this classification
06052020 CHE323-FS20-T2-20
Deduction of Mechanisms
44 Solvent effects
The solvent of a reaction may not only influence the rate
but also the overall mechanism path
in inorganic or organometallic reactions solvents often act as true ligands
solvents may stabilize destabilize ground ndash transition states or intermediates hellip
or they may be completely innocent
Fe FeOC
OC COCO
Fe FeOC
OC COCO
IFe
COCOI
+I2 +I- 2
Fe FeOC
OC COCO
Fe
COCOI
2
CHCl3
hexane
-+ -+ -+-
+
A B DC
06052020 CHE323-FS20-T2-21
Deduction of Mechanisms
44 Solvent effects
solvents may provide a solvent cage as the reaction vessel
the longer the reactants reside the faster the reaction
cage effect
contact ion-pair ion-pair separatedby one solvent layer
ion-pair separated by solvent layers
separated ion-pairs
z1 = z2 = 1 equiv 357 pmz1 = z2 = 2 equiv 1428 pmz1 = z2 = 3 equiv 3213 pm
critical distances for ion-pair formation
strongly dependent on dielectric constant ε
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
][FeNCS][H
kk]][NCS[Fe][H
kkdt
]d[FeNCS 221
321
2+
+minus
minusminus+
+
+
+minus
+=
0dt
]d[FeNCS2=
+
][Hkk
][Hkk
][NCS]O)[Fe(H]FeNCSO)[(HK
21
21
ee362
e2
52
+minus
minus
+
minus+
+
+
+==
06052020 CHE323-FS20-T2-16
Deduction of Mechanisms
43 Microscopic Reversibility
the rate law
forward reaction backward reaction
after equilibrium is achieved
and K becomes pH dependent
which it is obviously not according to reaction equation
e2
1ee3
1 ][FeNCSk][NCS][Fek +minus
minus+ =
e
e2
2e
ee3
2 ][H][FeNCSk
][H][NCS][Fek +
+
minus+
minus+
=
2
2
1
1
kk
kkK
minusminus
==
06052020 CHE323-FS20-T2-17
Deduction of Mechanisms
43 Microscopic Reversibility
Without consideration of the individual steps pH dependence
conflict with thermodynamics
the problem is solved by considering the microscopic reversibility of all
individual equilibria involved separately
pH independent path
pH dependent path
hence as it should be
06052020 CHE323-FS20-T2-18
Deduction of Mechanisms
43 Microscopic Reversibility
a mechanism proposed once upon a time in square planar complexes
ligand substitution illustrative for microscopic reversibility
PtCl Cl
ClCl2-
PtCl Cl
ClCl 2-
PtCl Cl
ClClCl
PtCl Cl
ClCl
no mirror plane in the mechanism impedes the mr principle
+Cl- -Cl-
PtCl Cl
ClCl2-
PtCl Cl
ClClCl
PtCl Cl
ClCl 2-+Cl- -Cl-
instead
tbp transition state
06052020 CHE323-FS20-T2-19
Deduction of Mechanisms
44 Solvent effects
solvents
protic aprotic others
H-bondsweakly dipolar
H-bondshigh degreeof association
dipolar
stronglydipolarpolarizable
strongdipoles
dipoles e- pairdonors
aromaticspolar
aromaticapolar
apolar
try to find examples for this classification
06052020 CHE323-FS20-T2-20
Deduction of Mechanisms
44 Solvent effects
The solvent of a reaction may not only influence the rate
but also the overall mechanism path
in inorganic or organometallic reactions solvents often act as true ligands
solvents may stabilize destabilize ground ndash transition states or intermediates hellip
or they may be completely innocent
Fe FeOC
OC COCO
Fe FeOC
OC COCO
IFe
COCOI
+I2 +I- 2
Fe FeOC
OC COCO
Fe
COCOI
2
CHCl3
hexane
-+ -+ -+-
+
A B DC
06052020 CHE323-FS20-T2-21
Deduction of Mechanisms
44 Solvent effects
solvents may provide a solvent cage as the reaction vessel
the longer the reactants reside the faster the reaction
cage effect
contact ion-pair ion-pair separatedby one solvent layer
ion-pair separated by solvent layers
separated ion-pairs
z1 = z2 = 1 equiv 357 pmz1 = z2 = 2 equiv 1428 pmz1 = z2 = 3 equiv 3213 pm
critical distances for ion-pair formation
strongly dependent on dielectric constant ε
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
e2
1ee3
1 ][FeNCSk][NCS][Fek +minus
minus+ =
e
e2
2e
ee3
2 ][H][FeNCSk
][H][NCS][Fek +
+
minus+
minus+
=
2
2
1
1
kk
kkK
minusminus
==
06052020 CHE323-FS20-T2-17
Deduction of Mechanisms
43 Microscopic Reversibility
Without consideration of the individual steps pH dependence
conflict with thermodynamics
the problem is solved by considering the microscopic reversibility of all
individual equilibria involved separately
pH independent path
pH dependent path
hence as it should be
06052020 CHE323-FS20-T2-18
Deduction of Mechanisms
43 Microscopic Reversibility
a mechanism proposed once upon a time in square planar complexes
ligand substitution illustrative for microscopic reversibility
PtCl Cl
ClCl2-
PtCl Cl
ClCl 2-
PtCl Cl
ClClCl
PtCl Cl
ClCl
no mirror plane in the mechanism impedes the mr principle
+Cl- -Cl-
PtCl Cl
ClCl2-
PtCl Cl
ClClCl
PtCl Cl
ClCl 2-+Cl- -Cl-
instead
tbp transition state
06052020 CHE323-FS20-T2-19
Deduction of Mechanisms
44 Solvent effects
solvents
protic aprotic others
H-bondsweakly dipolar
H-bondshigh degreeof association
dipolar
stronglydipolarpolarizable
strongdipoles
dipoles e- pairdonors
aromaticspolar
aromaticapolar
apolar
try to find examples for this classification
06052020 CHE323-FS20-T2-20
Deduction of Mechanisms
44 Solvent effects
The solvent of a reaction may not only influence the rate
but also the overall mechanism path
in inorganic or organometallic reactions solvents often act as true ligands
solvents may stabilize destabilize ground ndash transition states or intermediates hellip
or they may be completely innocent
Fe FeOC
OC COCO
Fe FeOC
OC COCO
IFe
COCOI
+I2 +I- 2
Fe FeOC
OC COCO
Fe
COCOI
2
CHCl3
hexane
-+ -+ -+-
+
A B DC
06052020 CHE323-FS20-T2-21
Deduction of Mechanisms
44 Solvent effects
solvents may provide a solvent cage as the reaction vessel
the longer the reactants reside the faster the reaction
cage effect
contact ion-pair ion-pair separatedby one solvent layer
ion-pair separated by solvent layers
separated ion-pairs
z1 = z2 = 1 equiv 357 pmz1 = z2 = 2 equiv 1428 pmz1 = z2 = 3 equiv 3213 pm
critical distances for ion-pair formation
strongly dependent on dielectric constant ε
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-18
Deduction of Mechanisms
43 Microscopic Reversibility
a mechanism proposed once upon a time in square planar complexes
ligand substitution illustrative for microscopic reversibility
PtCl Cl
ClCl2-
PtCl Cl
ClCl 2-
PtCl Cl
ClClCl
PtCl Cl
ClCl
no mirror plane in the mechanism impedes the mr principle
+Cl- -Cl-
PtCl Cl
ClCl2-
PtCl Cl
ClClCl
PtCl Cl
ClCl 2-+Cl- -Cl-
instead
tbp transition state
06052020 CHE323-FS20-T2-19
Deduction of Mechanisms
44 Solvent effects
solvents
protic aprotic others
H-bondsweakly dipolar
H-bondshigh degreeof association
dipolar
stronglydipolarpolarizable
strongdipoles
dipoles e- pairdonors
aromaticspolar
aromaticapolar
apolar
try to find examples for this classification
06052020 CHE323-FS20-T2-20
Deduction of Mechanisms
44 Solvent effects
The solvent of a reaction may not only influence the rate
but also the overall mechanism path
in inorganic or organometallic reactions solvents often act as true ligands
solvents may stabilize destabilize ground ndash transition states or intermediates hellip
or they may be completely innocent
Fe FeOC
OC COCO
Fe FeOC
OC COCO
IFe
COCOI
+I2 +I- 2
Fe FeOC
OC COCO
Fe
COCOI
2
CHCl3
hexane
-+ -+ -+-
+
A B DC
06052020 CHE323-FS20-T2-21
Deduction of Mechanisms
44 Solvent effects
solvents may provide a solvent cage as the reaction vessel
the longer the reactants reside the faster the reaction
cage effect
contact ion-pair ion-pair separatedby one solvent layer
ion-pair separated by solvent layers
separated ion-pairs
z1 = z2 = 1 equiv 357 pmz1 = z2 = 2 equiv 1428 pmz1 = z2 = 3 equiv 3213 pm
critical distances for ion-pair formation
strongly dependent on dielectric constant ε
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-19
Deduction of Mechanisms
44 Solvent effects
solvents
protic aprotic others
H-bondsweakly dipolar
H-bondshigh degreeof association
dipolar
stronglydipolarpolarizable
strongdipoles
dipoles e- pairdonors
aromaticspolar
aromaticapolar
apolar
try to find examples for this classification
06052020 CHE323-FS20-T2-20
Deduction of Mechanisms
44 Solvent effects
The solvent of a reaction may not only influence the rate
but also the overall mechanism path
in inorganic or organometallic reactions solvents often act as true ligands
solvents may stabilize destabilize ground ndash transition states or intermediates hellip
or they may be completely innocent
Fe FeOC
OC COCO
Fe FeOC
OC COCO
IFe
COCOI
+I2 +I- 2
Fe FeOC
OC COCO
Fe
COCOI
2
CHCl3
hexane
-+ -+ -+-
+
A B DC
06052020 CHE323-FS20-T2-21
Deduction of Mechanisms
44 Solvent effects
solvents may provide a solvent cage as the reaction vessel
the longer the reactants reside the faster the reaction
cage effect
contact ion-pair ion-pair separatedby one solvent layer
ion-pair separated by solvent layers
separated ion-pairs
z1 = z2 = 1 equiv 357 pmz1 = z2 = 2 equiv 1428 pmz1 = z2 = 3 equiv 3213 pm
critical distances for ion-pair formation
strongly dependent on dielectric constant ε
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-20
Deduction of Mechanisms
44 Solvent effects
The solvent of a reaction may not only influence the rate
but also the overall mechanism path
in inorganic or organometallic reactions solvents often act as true ligands
solvents may stabilize destabilize ground ndash transition states or intermediates hellip
or they may be completely innocent
Fe FeOC
OC COCO
Fe FeOC
OC COCO
IFe
COCOI
+I2 +I- 2
Fe FeOC
OC COCO
Fe
COCOI
2
CHCl3
hexane
-+ -+ -+-
+
A B DC
06052020 CHE323-FS20-T2-21
Deduction of Mechanisms
44 Solvent effects
solvents may provide a solvent cage as the reaction vessel
the longer the reactants reside the faster the reaction
cage effect
contact ion-pair ion-pair separatedby one solvent layer
ion-pair separated by solvent layers
separated ion-pairs
z1 = z2 = 1 equiv 357 pmz1 = z2 = 2 equiv 1428 pmz1 = z2 = 3 equiv 3213 pm
critical distances for ion-pair formation
strongly dependent on dielectric constant ε
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
-+ -+ -+-
+
A B DC
06052020 CHE323-FS20-T2-21
Deduction of Mechanisms
44 Solvent effects
solvents may provide a solvent cage as the reaction vessel
the longer the reactants reside the faster the reaction
cage effect
contact ion-pair ion-pair separatedby one solvent layer
ion-pair separated by solvent layers
separated ion-pairs
z1 = z2 = 1 equiv 357 pmz1 = z2 = 2 equiv 1428 pmz1 = z2 = 3 equiv 3213 pm
critical distances for ion-pair formation
strongly dependent on dielectric constant ε
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-22
Deduction of Mechanisms
44 Solvent effects
residence time in the cage given by the rate of diffusion
in water typically 10-12 bis 10-11 sec time to collide 10 ndash 100 times
consequence short lived intermediates can not be detected
since the immediately react further
the entity of compounds in a cage is called encounter complex
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Reactands activated compound increased polarity
SN1 RmdashX Rδ+middotmiddotmiddotmiddotmiddotXδ- ++SN1 RmdashX+ Rδ+middotmiddotmiddotmiddotmiddotXδ+ -SN2 Y + RmdashX Yδ+middotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- ++SN2 Y- + RmdashX Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ- -SN2 Y + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ -SN2 Y- + RmdashX+ Yδ-middotmiddotmiddotmiddotmiddotmiddotRmiddotmiddotmiddotmiddotmiddotmiddotXδ+ --
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-23
Deduction of Mechanisms
44 Solvent effects
the solvation of the encounter complex strongly contributes to ∆Sne
Reaction Charge product ZAZB
∆SDagger(J Kminus1 molminus1)
measured calculated
S2O32- + SO3
2- 4+ -126 -170
[ML5X]2+ + Hg2+ 4+ -60 to -104 -170
BrCH2CO2- + S2O3
2- 2+ -71 -85
ClCH2CO2- + OH- 1+ -50 -40
BrCH2CO2ME + S2O32- 0 +25 0
[Co(NH3)5Br]2+ + OH- 2- +92 +85
[Co(NH3)5(H2O)]3+ + Cl- 3- +142 +125
ReCl62- + Hg2+ 4- +142 +170
ML5= Cr(OH2)5 Cr(NH3)5 Co(NH3)5 Rh(NH3)5
How to interpret these data with respects to the solvent water
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
B + SbCl5 B-SbCl5 DZ(SbCl5) = -∆H
Drago und Weyland -∆H = EaEb + CaCb
Drago concept doesnt consider hardsoft (electrostatic ndash covalent)
06052020 CHE323-FS20-T2-24
Deduction of Mechanisms
44 Solvent effects
donoracceptor numbers are an indication for the behaviour of solvents
Gutmann
useful to compare relative characters of solvents
no obvious relationship to ε
new definition
E = electrostatic C = covalent a=acid b=base
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-25
Drago und Weyland -∆H = EaEb + CaCbnew definition
Deduction of Mechanisms
44 Solvent effects
I2 + Pyridin I2middotC5H5N
-∆Hber = (05 middot178) + (20 middot 354) = 797 kcalmol (-∆Hexp = 78 kcalmol)
acidic solvents which tend to interact electrostatically (large Es)
interact preferentially with bases with large Eb
and
acids which prefer covalent interaction (large Ca) interact
with bases of large Cb
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-26
Deduction of Mechanisms44 Solvent effects Drago-Weyland Donor and Acceptor numbers
Saumlure Es Cs Rs Saumlure Es Cs Rs
I2 050 200 --- H+ 4500 1303 13021
H2O 154 013 020 CH3+ 1970 1261 5509
SO2 056 152 085 Li+ 1172 145 2421
HFb 203 030 047 K+b 378 010b 2079
HCNb 177 050 054 NO+b 01b 686 4599
CH3OH 125 075 039 NH4+b 431 431 1852
H2Sb 077 146 056 (CH3)2NH2+
b321 070 2072
HCIb 369 074 055 (CH3)4N+b 196 236 833
C6H5OH 227 107 039 C5H5NH+ 181 133 2172
(CH3)3COH 136 051 048 (C2H5)3NH+b
243 205 1181
HCCI3 149 046 045 (CH3)3NH+b 260 133 1595
CH3CO2Hb 172 086 063 H3O+ 1327 789 2001
CF3CH2OH 207 106 038 (H2O)2H+ 1139 603 736
C2H5OH 134 069 041 (H2O)3H+ 1121 466 234
i-C3H7OH 114 090 046 (H2O)4H+b 1068 411 325
PF3b 061 036 087 (CH3)3Sn+ 705 315 2693
B(OCH3)3b 054 122 084 (C5H5)Ni+ 1188 349 3264
AsF3b 148 114 078 (CH3)NH3
+b 218 238 2068
Fe(CO)3b 010 027 100
CHF3b 132 091 027
B(C2H5)3b 170 271 061
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-27
Deduction of Mechanisms
Basec EB CB TB Basec EB CB TB
NH3 231 204 056 C5H5NO 229 233 067
CH3NH2 216 312 059 (CH3)3P 146 344 090
(CH3)2NH 180 421 064 (CH3)2O 168 150 073
(CH3)3N 121 561 075 (CH3)2S 025 375 107
C2H5NH2 235 330 054 CH3OH 180 065 070
(C2H5)3N 132 573 076 C2H5OH 185 109 070
HC(C2H4)3N 080 672 083d C6H6 070 045 081
C5H5N 178 354 073 H2Sb 004 156 113
4-CH3C5H4N 174 393 073d HCNb 119 010 090
3-CH3C5H4N 176 372 074d H2COb 156 010 076
3-ClC5H4N 178 281 075d CH3Clb 254 010 023
CH3CN 164 071 083 CH3CHOb 176 081 074
CH3C(O)CH3 174 126 080 H2Ob 228 010 043
CH3C(O)OCH3 163 095 086 (CH3)3COHb 192 122 071
CH3C(O)OC2H5 162 098 089 C6H5CNb 175 062 085
HC(O)N(CH3)2 219 131 074d F- 973 428 3740
(C2H5)2O 180 163 076 Cl-b 750 376 1230
O(CH2CH2)2O 186 129 071 Br-b 674 321 586
(CH2)4O 164 218 075 I- 548 297 626
(CH2)5O 170 202 074d CN- 723 652 920
(C2H5)2S 024 392 110d OH-b 1043 460 5073
(CH3)2SO 240 147 065 CH3O-b 1003 442 3377
44 Solvent effects Drago-Weyland Donor and Acceptor numbers
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
CH3COOH is an acid in water and an electrolyte
CH3COOH in H2SO4 is a base base strength levelled by [HSO4]-
06052020 CHE323-FS20-T2-28
Deduction of Mechanisms
44 Acids as solvents and super acids
most Broslashnsted acids show auto dissociation like water
pH-scale and value defined by the degree of auto dissociation
therefore each acid has its individual pH-scale
all acidbase pairs which are inside these limits act as electrolyte
all acidbase pairs outside these limits will be levelled by the acid solvent
more practical dissolution of amide H2N- is levelled by base strength of [OH]-
since outside the pH scale of water
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
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Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-29
H2 O
MeO
H
EtOH
HC
ON
H2
DM
A
DM
F
DM
SO
TMS
H3 C
NO
2
H3 C
CN
THF
DM
E
acetone
HM
PT
py
NH
3 NM
P
ETA
HC
O2 H
N2 H
4
H2 SO
4
HF
H3 C
CO
2 H
F3 C
CO
2 H
0
14
4867
21 20 177
14
1-1 1 1
-7-103
-52-37
13 132 1310
85 63
-16-14
-28
-20
-16
-7-5
187
262
1
4
-8
23275
16
3430
41
31
Super acid media
Super base media
0 01
4030
2010
-40-20
-10-30
2-1
-2-3
E (V) vs NH
E
R(H
+ )
Deduction of Mechanisms
44 Acids as solvents and super acids
R(H) potentiometric acidity functionStrehlow-function
B Treacutemillon D Inman Reactions in Solution An Applied Analytical Approach Wiley 1997 pp 227-300
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
H0 = pKBH+ - log B = nitroanilin indicator[BH+][B]
H2SO4 -119HF -110HClO4 -13HSO3F -156HSO3FSbF5 -210 bis -250
H2SO4 + H2S2O7 [H3SO4]+ + [HS2O7]-
Pyroschwefelsaumlure
super acids H0
06052020 CHE323-FS20-T2-30
Deduction of Mechanisms
44 Acids as solvents and super acids
acids which are more acidic than sulfuric acid are called super acids
Hammett acidity function
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
HFSbF5 -210 bis -280
2 HSO3F H2SO3F+ + SO3F-
H[Sb2F11] H[Sb2F10(SO3F)]
2 SbF52 SbF5
very strong acids in H2SO4
H3BO3 + H2SO4 3 [H3O]+ + B(HSO4)4]- + 2 [HSO4]-
with SO3 [H3O]+ and [HSO4]- is removed
3 [H3O]+ + [B(HSO4)4]- + 2 [HSO4]- [H3SO4]+ + [B(HSO4)4]- + 4 H2SO4+ 2 SO3
bdquoMagic acidsldquo
2 HF H2F+ + F-
06052020 CHE323-FS20-T2-31
Deduction of Mechanisms
44 Acids as solvents and super acids
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
- the strongest known acids are based on Carborane anions
eg Ch Reed Angew Chem Int Ed 2004 43 5352
H-LB + LS-A LS-LB + H-A
extrem starke LS Silylkationen R3Si+
starke LB Cl-
A extrem schwaches Nucleophil
06052020 CHE323-FS20-T2-32
Deduction of Mechanisms
44 Acids as solvents and super acids
the conjugate base of the super acid may not coordinate or oxidize
need for delocalization over many atoms
principle behind synthesis Broslashnsted acidity induced by Lewis acidity
a classical hard-soft problem
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
Carborane anions are extremely acidic and not oxidizing
[CB11H13] [CB11H12]- + H+
[CB11H12]-ICl Cl2 [CB11H6X6]-
06052020 CHE323-FS20-T2-33
Deduction of Mechanisms
44 Acids as solvents and super acids
the H+ can only be introduced via LA-LB interactions
[Ph3C][CB11H6X6] + [R3Si-H] [R3Si-CB11H6X6] + Ph3C-H
[R3Si-CB11H6X6] + HCl [H-CB11H6X6] + R3Si-Cl
formal R3Si+ cation
super-super acid
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-34
Deduction of Mechanisms
44 Acids as solvents and super acids
what can we do with super-super acids
AuF3 + 6Xe + 3H+ [AuXe4]2+ + [Xe2]2+ + 3HFHFSbF5
mechanistic principle super-super acid protonates AuF3
AuF3 + 3H+ [Au(FH)3]3+
very strong oxidant
2[Au(FH)3]3+ + 6Xe [AuXe4]2+ + [Xe2]2+ + 3HF
[Sb2F11]- is the counter-ion
S Seidel K Seppelt Science 2000 290 117T Drews K Seppelt Angew Chem Int Ed1997 36 273
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-35
Deduction of Mechanisms
44 Acids as solvents and super acidsProtonation of CO and generation of the formyl cation HCO+
CO CO(sol)[H2F]+[SbxF5x+1]-
C O
F
H
SbF5 [HCO]+[SbF6]-
[HCO]+[SbxF5x+1] + HF
Variable-temperature1H 13C and 19F HP NMR spectra of 13CO
(538 mmol) in HF-SbF5 (164 mmol) in a sapphire NMR tube
(total pressure 26 atm at 25degC) The broad resonance at δ(19F) =
minus113 (Table 1) is not shown in the 19F spectrum
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
[H-CB11H6X6] + C60 [CB11H6X6][HC60]
[H-CB11H6X6] + C6H6
benzene derivatives
[CB11H6X6][C6H7]
06052020 CHE323-FS20-T2-36
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
fullerene
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
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[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-37
Deduction of Mechanisms
44 Acids as solvents and super acids
Beside CO super acids also protonate alkanes but also small molecules like N2 Ar ao
protonation frequently under alkane elimination makes sa synthetically useful
a tbutyl cation which can be stored in bottles
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
ABABdc DrNπ4000k sdotsdotsdotsdot=
the resulting rate constant is kdc sim 6sdot109 M-1 sdotsec-1
rAB is the reaction cross section DAB = DA + DB the diffusion constant
a typical range for DAB is 2sdot10-9 m2sdotsec-1 for rA 02 nm or 2sdot10-10 m
depends on the viscosity of the solvent
06052020 CHE323-FS20-T2-38
if the rate of a reaction is given by the collision probability it runs
at maximum speed
such a reaction runs at a diffusion controlled rate
the frequency is defined by the Fick law of diffusion
the relevant relationship is (Smoluchowski)
Deduction of Mechanisms
45 Diffusion controlled reactions
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
kdc does not depend significantly on the reaction partners
mit η = Viskositaumlt
n-Pentane 215 31middot1010
Diethyl ether 222 30middot1010
Acetone 316 21middot1010
Benzene 603 11middot1010
Water 898 74middot109
Acetic acid 116 57middot109
Benzonitrile 145 46middot109
Ethylene glycol 136 49middot108
Cyclohexanol 410 16middot108
Glycerol 9450 70middot106
Solvent η10-4 kg m-1 s-1 kdcL mol-1 s-1
η3TR0008kdcsdotsdot
geif vA sim vB
resulting activation energy from kdc Ea sim 4 - 20 kJmol
06052020 CHE323-FS20-T2-39
kBmiddotT6middotπmiddotrAmiddotη
DA =
Deduction of Mechanisms
45 Diffusion controlled reactions
a big molecule diffuses slower but may have a larger reaction cross section
for other solvents the diffusion constant can be calculated according to Einstein-Stokes
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-40
Deduction of Mechanisms
45 Diffusion controlled reactions
example for a rapid reaction
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-41
12 ps
10 smicro
10 smicro
100 ns
21 ns
398 ns
1900 1950 2000 2050Wavenumberscm-1
Abso
rban
ceAb
sorb
ance
Cha
nge
10 mOD
a
b
Deduction of Mechanisms
45 Diffusion controlled reactions
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
2mm
H
D
DR
HR ==minus
minus
νν
If a reaction rate depends on the isotope (HD)
then this step is rate determining
if force constants for R-H and R-D are equal
06052020 CHE323-FS20-T2-42
Deduction of Mechanisms
46 Kinetic Isotope Effect
Exchanging isotopes is often a good hint to a reaction mechanism and
the relevance of individual elementary steps
An isotopic effect is generally observable with HD
but is very difficult to detect with other isotopes eg12C13C
vibrational energy of a molecule En=(n+12)sdothsdotν
harmonic oscillator =1
2π119896119896119896μν
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
from Arrhenius
minus
= minusminus
2RT)N(hexp
kk DRHR
D
H νν
and for R = C follows that kHkD = 78
C-H 2900 78O-H 3300 103 N-H 3100 89 S-H 2600 63
Bond νHcm-1 kHkD
H0
D0
Da
Ha EEEE minus=minus
06052020 CHE323-FS20-T2-43
Deduction of Mechanisms
46 Kinetic Isotope Effect
Principle of kie Difference of activation energy is equal to the difference of
ground state energies since R-H(D) bonds must be broken
accordingly
or AHAD
∆ERsdotT-ln=
kHkD
ln
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
und nicht
Beispiel Nitrierung von C6H6 zeigt keinen Isotopeneffekt
06052020 CHE323-FS20-T2-44
NO2 + H+NO2
+ + k
H NO2
k
NO2
NO2+
+ H+
H
Deduction of Mechanisms
46 Kinetic Isotope Effect
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-45
Deduction of Mechanisms
46 Kinetic Isotope Effect
the following reaction has a kie of 045 why
[PtH2(PMe3)2] [Pt(PMe3)2] + H2
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-46
Deduction of Mechanisms
46 Kinetic Isotope Effect
How thermodynamics influences the magnitude of the kie
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-47
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
MA Sierra et al Chem Rev 2011 111 4857
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-48
Deduction of Mechanisms
46 Kinetic Isotope Effect
a few examples for normal and inverted kies
what would you expect
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
∆Gne and ∆G are correlated although they have sometimes
nothing to do with each other
06052020 CHE323-FS20-T2-49
5 Linear Free Energy Relationships LFER
Kinetic rate laws describe the time course of a reaction
as a function of the elementary steps
a relationship between kinetics and thermodynamics of a reaction
is called a Linear Free Energy Relationship LFER
LFER are a semi quantitative way of ldquosystematizingrdquo
our ideas about the similarity of reactions
only sets of closely related reactions should be compared in that way
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-50
Linear Free Energy Relationships LFER
the most useful examples have well-defined domains of applicability such as
the Hammett equation
the Broslashnsted catalysis law
and the Marcus equation (see later)
only sets of closely related reactions should be compared in that way
an LFER is of the general form lnk = msdotlnKc + b since
lnk = lnksdotTh
∆GRsdotT-
lnKc =∆G0
RsdotT-∆G = m∆G0 + b
whereas the parameters m and b may tell us something about the mechanism
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
oder logk1 = logK + logk-11
1
kkKminus
=
(1) X- = F-
(2) X- = H2PO4-
(3) X- = Cl-(4) X- = Br-
(5) X- = I-(6) X- = NO3
-
If the process is dissociative and goes through a CN=5 then
k-1 must be constant due to the microscopic reversibility
thus we must have an LFER between log k and log K as a function of X
06052020 CHE323-FS20-T2-51
Linear Free Energy Relationships LFER
example
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-52
oder logk1 = logK + logk-11
1
kkKminus
=
Linear Free Energy Relationships LFER
example
hellip and vice versa if the mechanism is associative CN=7 in transitions state
we must find a LFER between logk-1 vs logK
NH3
Co
NH3
NH3
X
H3N
H3N
2+
k1
K
NH3
Co
NH3
NH3
OH2
H3N
H3N
3+
k-1
frequently logK is not known then a correlation can also be made
with a similar physico-chemical parameter
eg in this case with the pKa values or with NMR shifts
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-53
Linear Free Energy Relationships LFER51 Hammett Correlation
This relationship correlates the influence of mp-substituents in aromatic systems
(originally) to the hydrolysis of benzoic-acid esters
O-
O
XOEt
O
X + EtOHk1
kinetics
OH
O
XK1
thermodynamics
O-
O
X + H+
LFER logk1 ndash logk0 = ρsdot(logKa ndash logKa0)
or log k1k1
0KaKa
0ρsdotlog= and Ka
Ka0
log =σ
σ= Hammett Parameter
hundreds of σ-values are available see eg RW Taft et al Chem Rev 1991 91 165
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
Linear Free Energy Relationships LFER51 Hammett Correlation
negative σ-values stand for donating positive σ-values for accepting groups
inspection of curvature eg k1 vs σ gives an insight into the type of mechanism
Hammett equation has been modified to include
steric inductive or resonance effects (Taft)
Example from organometallic chemistry disulfide elimination
rate=ksdot[CO]sdot[(NHC)(SPh)Fe(NO)2]
∆H=78 kcalmol ∆S=-45 eu
MY Darensbourg et al Chem Sci 2014 5 3795
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-55
Linear Free Energy Relationships LFER51 Hammett Correlation
correlation of νNO vs Hammett Parameter σ
E12 vs σ
rate retardation by e- withdrawing substituents
in agreement with mechanism
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-56
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
M - X M+ + X-
M+ + Y- M-Y
Pro memoria Classification of substitution mechanisms
Hughes-Ingold SN1 - Langford-Gray D
M-X + Y M -- X -- Y M-Y + X
Hughes-Ingold SN1 - Langford-Gray Id Ia
M-X + Y Y -- M -- X M-Y + X
Hughes-Ingold SN2 - Langford-Gray A
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-57
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
A or D mechanisms may show different LFER the LFSE plays a role
nomenclature- dissociative activation Id influence of leaving group
- associative activation Ia influence of entering group
intimate mechanisms if in between
must be elucidated from a series of experiments eg with LFER relations
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
Mechanisms Crystal Field Activation Approach (CFSE)
Assumption changes in transition GS TS determines mechanism
7-coordinatepentagonal bipyramid D5h
7-coordinateoctahedralwedgeC2v
6-coordinateoctahedron Oh
5-coordinatesquarepyramidC4v
5-coordinatetrigonalbipyramidD3h
0
4
8
-4
-8
Dq
06052020 CHE323-FS20-T2-58
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
d0 0 0 0 0d1 128 -057 -208 128d2 256 -114 -068 -256d3 575 200 180 426d4 low spin 702 143 -026 298d4 high spin -108 -314 -279 107d5 low spin 830 086 114 170d5 high spin 0 0 0 0d6 low spin 1148 400 363 852d6 high spin 027 -057 -208 -128d7 low spin 466 -114 -098 534d7 high spin 255 -114 -068 -256d8 573 200 180 426d9 -108 314 -279 107d10 0 0 0 0
Symmetry transition state D3h C4v C2v D5h
Mode of activation dissociative associative
06052020 CHE323-FS20-T2-59
Mechanisms Crystal Field Activation Approach (CFSE)
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-60
Linear Free Energy Relationships LFER52 Ligand Field Stabilization Energies
d3 d6 (ls) d8 inert centres
d4 d7 d9 gain energy in other states
Crystal field activation energy (CFAE) is (almost) always positive
CFAE is zero for d0 d5 hs d10 (if we dont have a spin crossover)
Jahn-Teller states
capped octahedral favored(KZ = 7)
Analyse von CFAE is not quantitative but enables a certain prediction
square pyramid favoured (KZ = 5)
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
Mn(-I) Fe(0) Co(I) Ni(II) Cu(III)Rh(I) Pd(II) Ag(III)Ir(I) Pt(II) Au(III)
Pd2+ about 105 time more reactive than Pt2+
Classical reaction
trans-[PdCl2(SMe2)2]cis-[PdCl2(SMe2)2]
v = k1sdot[PdCl2(Me2S)2]sdot[Me2S]
and
trans-[PtCl2(SMe2)(SMe2)] + Me2Strans-[PtCl2(SMe2)2] + Me2S
v = k1sdot[PtCl2(Me2S)2]sdot[Me2S] 06052020 CHE323-FS20-T2-61
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
square planar specially stabilized are d8 systems
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
T
X
L
L
TL
XL
ST
S
L
L
T L
SL
Y
Y
+ Y-
T
Y
L
L
- s
T L
XL
Y - x
s - x
Associatively driven reaction often in combination with the solvent
schnelles Gleichgewichtoder
steady state
k2
k1k-1
k3
06052020 CHE323-FS20-T2-62direct solvent mediated
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
[L3M-X] + [sol] [L3M-sol] + [X]
[L3M-sol] + [Y] [L3M-Y]
k1
k-1 k2
steady state assumption for solvent mediated path
dPdt =
k1sdotk2sdot[L3M-X]sdot[Y]k-1sdot[X] + k2sdot[Y]
if k2 very fast v = k1sdot[L3M-X]if [X] very large suppression of this path
direct path k3sdot[L3M-X]sdot[Y]dPdt =
=dPdt k3sdot[L3M-X]sdot[Y] + k1sdot[L3M-X] with kobs = (k1 + k3sdot[Y])
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
06052020 CHE323-FS20-T2-63
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
[L3M-X] + [sol] [L3M-sol] + [X]Ksol
[L3M-sol] + [Y] [L3M-Y]k2
K = [L3M-sol]sdot[X][L3M-X] sdot[sol]
fB =
[L3M-X] = A [L3M-sol] = B [L3M-Y] = C
[B][A] + [B] =
Ksdot[sol]Ksdot[sol] + [X] [B] = fBsdot[A]
v = k2sdotKsdot[A]sdot[Y]sdot[sol]Ksdot[sol] + [X]
if K very large v = k2sdot[A][Y]if K small suppression of this path
06052020 CHE323-FS20-T2-64
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
[L3M-X] + [Y] [L3M-Y] + [X]Overall reaction
prior equilibrium assumption for solvent mediated path
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
L k1[s-1] k2 [l mol-1s-1]
CH3- 17 middot 10-4 67 middot 10-2
C6H5- 33 middot 10-5 16 middot 10-2
Cl- 10 middot 10-6 40 middot 10-4
H- 18 middot 10-2
PEt3 17 middot 10-2
v = kobs middot [PtCl(PEt3)2L]
Activation parameters for substitution in sp complexes
kobs = k1 + k2 middot [I-]
k1 k2
∆HDagger ∆SDagger ∆VDagger ∆HDagger ∆SDagger ∆VDagger
trans-[PtCl(NO2)(py)2] + py 50 -100 -38trans-[PtBr(Mes)(PEt3)2] + SC(NH2)2 71 -84 -46 46 -138 -54cis-[PtBr(Mes)(PEt3)2] + I- 84 -59 -67 63 -121 -63cis-[PtBr(Mes)(PEt3)2] + SC(NH2)2 79 -71 -71 59 -121 -54[AuCl(dien)]2+ + Br- 54 -17
Reaktion
Mes = 246-Trimethylphenyl Enthalpieterme in kJ mol-1 Entropie in J K-1 mol-1 Volumina in cm3 mol-106052020 CHE323-FS20-T2-65
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
Stereochemistry generally the relative configuration is maintained Retention
06052020 CHE323-FS20-T2-66
M
C
C X
T
+ YM
C
C
T
X
Y
MC X
T
C
Y
MC
X
T
C
Y
-X
MC
T
C
Y
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
5-coordinate transition state with enteringleaving groups in the eq plane
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-67
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
example is the cyanide exchange at [Pt(CN)4]2- proton mediated
MNC
NC CN
CNM
NC
NC CN
CN
2- CN
- 2-
F J Monlien et al Inorg Chem 2002 41 1717
dPdt = k[Pt(CN)4][CN]
substitution on d8 systems purely A
rate law
andor [Pt(CN)4]2- + H+ [Pt(CN)3(CNH)]-
[Pt(CN)3(CN)]-[Pt(CN)3(CNH)]- + CN-
[Pt(CN)3(CN)]-
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
Cyanide exchange [Pt(CN4)]2- is very stable logβ4 asymp 40
Exchange followed by NMR
pH = 60 [Pt] = 01 M [CN] = 026 M
Kinetics is purely 2nd order hellip
d[Pt]dt = k2[CN][Pt]
d[Pt]dt = k2[CN]0
KaKa + [H+]
rArr hellip and pH dependent
06052020 CHE323-FS20-T2-68
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
∆H = 25 kJmol∆S = -142 eu∆V = -27 cm3 mol-1
k2 = 11 sec-1M-1
06052020 CHE323-FS20-T2-69
Linear Free Energy Relationships LFER53 Substitution in square planar complexes
pure 2nd order means only direct exchange is important
protonation of [Pt(CN)4]2- does not play any role
why do we still see a linear increase
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-70
Linear Free Energy Relationships LFER54 trans-effect trans-influence
since substitution reactions on sp complexes are associative A
there might be a relationship between the free energy of activation
and the nucleophilic character of the entering group ie an LFER
These scales are metal dependent and require a zero point (as in Hammett)
[L3Pt-X] + [Z] [L3Pt-Z] + [X]
we compare two reactions
Z= zero here MeOHk1
[L3Pt-X] + [Y] [L3Pt-Y] + [X]k2
nucleophilicity for PtII
Y= entering ligand
reaction in MeOH is pseudo 1st order k10=k1[MeOH]
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-71
Linear Free Energy Relationships LFER54 trans-effect trans-influence
ηPt=k2k1
log scaled to η0Pt = ηPt + log243 = ηPt + 139
represents an LFER scale for the nucleophilicity of PtII
examples Carbon ηPt0 Nitrogen Sulfur
C6H11NC 634 NH3 307 PhSH 415
CN- 714 C5H5N 319 Et2S 452
halogens ηPt0 NO2
- 322 Me2S 487
F- lt22 N3- 348 SCN- 575
Cl- 304 NH2OH 385 (NH2)2CS 717
Br- 418 Phosphor ηPt0 Selenium
I- 546 PPh3 893 Me2Se 570
Et3P 899 SeCN- 711
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-72
54 trans-effect trans-influence
the influence of the non-participating ligands is called trans-effect or trans-influence
L
Pt
L
LGT + EG
L
Pt
L
EGT + LG
Linear Free Energy Relationships LFER
trans-directing influence of a ligand T
question is there a cis-directing mechanism
dxz pz
empty LUMO
filled HOMO
dp hybrids
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-73
dxz pz
empty LUMO
filled HOMO
dp hybrids
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans ligand
π
electron density from nucleophile
is stabilized by π-backbonding
the better the π-accepting properties of T the more stable the transition state
trans effect
trans effect
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-74
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
d or 2sdotp
trans influence
T LG
trans ligand T
LG
EG
strong M-T bond weakens
bond trans to it
ground state destabilization
trans influence
trans-effect and -influence are two different molecular phenomena with the same effect
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-75
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-influence weakens ground state binding energy of M-LG
mirrored by bond-lengthsPR3
Pt
PR3
ClT
trans-ligand PR3 Pt-Cl (Aring)
Cl- PEt3 2294
C2F5- PMePh2 2361
C6H5- PPh3 2408
CH3- PMePh2 2412
(H3C)3SiCH2- PMe2Ph 2415
H- PEtPh2 2422
Ph2MeSi- PMePh2 245
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-76
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
the trans influence mirrors an LFER since destabilization of ground state
directly translate into acceleration of substitution
trans influence goes about along with σ-donating properties ie nucleophilicity
CO lt H2O lt NH3 lt NH2R lt C2H4 lt Cl- lt Me2SO lt Br-
lt Me2S lt AsPh3 lt PPh3 lt CH3- lt H- lt Si(CH3) 3
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-77
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
to disentangle from the kinetic trans effect rate measurements are required
since the effect comes from the transition state stabilization
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-78
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
Cl
Pt
Cl
NT
Cl
Pt
Cl
OH-MeT N+
Trans-ligand k1 x 103 (s-1) k1 x 103 (s-1)
C2H4 6800 too fast
CO 1210 too fast
P(OMe)3 365 5240
PPh3 27 635
PEt3 26 495
PBu3 23 380
PMe3 15 275
AsEt3 17 39
Me2SO 01 28
trans effect series H2O lt NH3 lt Cl- lt Br- lt I- asymp [NO2]- lt Me2S lt Et2S lt Me2SO lt AsEt3lt Ph- lt Me- lt PPh3 lt PMe3 lt P(OMe)3asymp H- ltlt CO asymp CN-lt C2H4
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-79
54 trans-effect trans-influence
Linear Free Energy Relationships LFER
trans-effect useful for syntheses
+ NH3 + NH3
cis
Cl
Pt
Cl
ClCl
2- NH3
Pt
Cl
ClCl
- NH3
Pt
Cl
NH3Cl
-
NH3
Pt
NH3
NH3H3N
2+ NH3
Pt
Cl
NH3H3N
+ Cl
Pt
Cl
NH3H3N+ Cl- + NH3
trans
how to prepare
py
Pt
NO2
ClCl
NO2
Pt
CN
NH3NC
PR3
Pt
CO
PR3Cl
2+
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
Electron Transfer Reactions
06052020 ACIV-FS20-T2-80
Redox reactions are ubiquitous in chemistry
How are electrons transferred from the oxidant to the reductant
We discern two different type of electron transfer reactions
+
red ox
e-red+ ox
-
+
inner sphere mechanism
+
red ox
e-red+ ox
-
outer sphere mechanism
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 ACIV-FS20-T2-81
Electron Transfer Reactions
Taubes famous Experiment
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
puzzling substitution rate Cl- for H2O in A is 2middot10-6 s-1
A B
equally puzzling substitution rate of H2O for Cl- in B is 3middot10-8 s-1
Cl- can not bind to CrIII after the redox process
only explanation Cl- must bridge the two centres before and during e--transfer
inner sphere electron transfer
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 ACIV-FS20-T2-82
Electron Transfer Reactions
CoH3N
H3N NH3
Cl
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrClH2O OH2
OH2
OH2
OH2
2+ 2+
+ 5 NH4+
rate constant of overall reaction 6middot105 mol-1middots-1 (t12asymp 1 ms)
A B
compare to
CoH3N
H3N NH3
NH3
NH3
NH3
+ CrH2O
H2O OH2
OH2
OH2
OH2
3+ 2+
CoH2O
H2O OH2
OH2
OH2
OH2
+ CrH2O
H2O OH2
OH2
OH2
OH2
2+ 3+
+ 6 NH4+
rate constant of overall reaction 10-3 mol-1middots-1 (t12asymp )
does not mean that outer sphere reactions are agrave priori slower
Inner sphere electron transfer reactions
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 ACIV-FS20-T2-83
Electron Transfer Reactions Inner sphere electron transfer reactions
individual steps in is mechanisms
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-B-MYL5
L5MX-1-B-MY+1L5
L5MX-1-OH2B-MY+1L5L5MX-1-BH2O-MY+1L5
L5MX-1-OH2 + B-MY+1L5L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
bridge formation kB
emdashtransfer adiabatic
bridge rupture
separation
self exchange rates are relevant
products
educts
or
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 ACIV-FS20-T2-84
encounter complex bridge formation rupture of bridge(after e--transfer)
Electron Transfer Reactions Inner sphere electron transfer reactions
reaction profiles with different rate determining steps
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 ACIV-FS20-T2-85
Electron Transfer Reactions Inner sphere electron transfer reactions
rate law for an inner sphere mechanism
v = kbmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
ket is included in kb since adiabatic (extremely fast)
CoH3N
H3N NH3
N
NH3
NH3
3+
CN
CrH2O
H2O OH2
OH2
OH2
OH2
2+
CrH2O
H2O OH2
NC
OH2
OH2
3+
NCo
H2O
H2O OH2
OH2
OH2
OH2
2+
+ +
kinetics
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 ACIV-FS20-T2-86
Electron Transfer Reactions Inner sphere electron transfer reactions
very strong bridging ligand dependenceoxidant k (M-1s-1) ∆S[Co(NH3)5F]2+ 25middot105
[Co(NH3)5Cl]2+ 60middot105
[Co(NH3)5Br]2+ 14middot106
[Co(NH3)5I]2+ 30middot106
[Co(NH3)5OH]2+ 15middot106
[Co(NH3)5(OH2)]3+ 01
[Co(NH3)5(NCS)]2+ 19 (remote) -121
[Co(NH3)5(SCN)]2+ 19middot105
[Co(NH3)5(O2CH)]2+ 72 -113
[Co(NH3)5(O2CCH3)]2+ 035 -138
[Co(NH3)5(O2CC(CH3)3)]2+ 0007 -130
[Co(NH3)5(O2CCH2NH3]3+ 0064
[Co(NH3)5(O2CNH2]2+ 242 -78
[Co(NH3)5(OC(S)NHCH3]2+ 68
[Co(NH3)5(SC(O)NHCH3]2+ 65middot104
(red = [Cr(OH2)6]2+)
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 ACIV-FS20-T2-87
Electron Transfer Reactions Inner sphere electron transfer reactions
criteria for inner sphere reactions
reasonable bridging ligands (typically with a lone pair)
conductive ligands
if the e--transfer is equal or slower than rates of substitution
(if rates of e--transfer are faster than eg ligand self exchange then it must be os)
if reactions are faster than predicted by Marcus-Hush correlation (see later)
HSAB aspects
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 ACIV-FS20-T2-88
Electron Transfer Reactions Outer sphere electron transfer reactions
+
red ox
e-red+ ox
-
outer sphere mechanism
classical experiment ∆-[Os(bpy)3]3+ does not racemize at all
addition of Λ- [Os(bpy)3]2+ leads to very rapid racemization
how does this work
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 ACIV-FS20-T2-89
Electron Transfer Reactions Outer sphere electron transfer reactions
L5MX-B + L5MY-OH2
L5MX-BH2O-MYL5
L5MX-1-B + H2O-MY+1L5
Kpeformation of encounter complex
e--transfer kET
separation
educts
L5MX-1-BH2O-MY+1L5
products
rate laws follow a typical Langford-Gray mechanism
v = kETmiddotKpemiddot[MX]middot[MY]
1+Kpemiddot[MY]
much simpler than inner sphere
kobs = kETmiddotKpemiddot [MY]
1+Kpemiddot[MY]
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 ACIV-FS20-T2-90
Electron Transfer Reactions Outer sphere electron transfer reactions
this relation ship leads to typical saturation kinetic behaviour
[Co(NH3)5OH2]3+ + [Fe(CN)6]4- [Co(NH3)5OH23+][Fe(CN)6
4-]example
KOS = 1500M-1
fast
activation ndash electron transfer - separation
[Co(NH3)5OH23+][Fe(CN)6
4-] [Co(NH3)5OH22+][Fe(CN)6
3-][Co(OH2)6]2++ [Fe(CN)6]3-kET = 19middot10-1 s-1 t12 = 4 sec
encounter complex formation
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 ACIV-FS20-T2-91
Electron Transfer Reactions Outer sphere electron transfer reactions
The charges of the encounter complex is essential for the overall rate
Ox Red KOS ket (s-1)[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 1500 019
[Co(NH3)5(py)]3+ 2400 0015
[Co(NH3)5(bpy)]3+ 2300 0024
[Co(NH3)5(dmso)]3+ [Fe(CN)5L]3-
L=Imidazol 450 26
L=NH3 420 13
L=Pyridine 490 015
L=Isonicotinamid 600 005
L=pyrazine 360 00089
[Co(NH3)5(ac)]2+ [Fe(CN)6]4- 300 000037
[Co(NH3)5(benzoat)]2+ 240 000062
[Co(NH3)5(Cl)]2+ 38 0027
[Co(NH3)5(N3)]2+ 49 000062
12-
9-
8-
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
Electron Transfer Reactions Outer sphere electron transfer reactions
is ket dependent of the driving force ie ∆Edeg
L5MX-BH2O-MYL5 ketL5MX-1-BH2O-MY+1L5
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
Electron Transfer Reactions Outer sphere electron transfer reactions
some more activation parameters for outer sphere reactions
ket (s-1) ∆Hne ∆Sne ∆Vne
[Co(NH3)5(OH2)]3+ [Fe(CN)6]4- 12middot10-1 102 79 265
[Co(NH3)5py]3+ 93middot10-2 118 113 298
[Co(NH3)5(OSMe2)]3+ 20middot10-1 84 25 344
[Co(NH3)5N3]2+ 62middot10-4 104 44 188
[Co(NH3)5Cl]2+ 27middot10-2 85 11 259
[Co(phen)3]2+ 24middot10-1 32 -148
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 ACIV-FS20-T2-94
[Co(bpy)3]3+ [Cr(phen)3]2+ 20middot108
[Ru(bpy)3]3+ [Ru(NH3)6]2+ 37middot109
[Co(phen)3]3+ [Co(terpy)2]2+ 42middot102 276 -100[Co(phen)3]3+ [Ru(NH3)6]2+ 15middot104 18 -105[Co(bpy)3]3+ [Co(terpy)2]2+ 30middot101 21 -155[Co(phen)3]3+ [Ru(NH3)5py]2+ 19middot103 21 -100[Co(NH3)6]3+ [Ru(NH3)6]2+ 11middot10-2 56 -1[Fe(H2O)6]3+ [Ru(NH3)5py]2+ 78middot104 20 -84[Ru(NH3)5py]3+ [V(H2O)6]2+ 30middot105 0 -138[Co(phen)3]3+ [V(H2O)6]2+ 40middot103 16 -121[Co(terpy)2]3+ [V(H2O)6]2+ 38middot103 79 -150[Co(NH3)6]3+ [Cr(H2O)6]2+ 10middot10-3
[Ni(bpy)3]3+ [Fe(H2O)6]2+ 67middot106 7 -92[Co(en)3]3+ [V(pic)3]- 31middot103 52 -4[Fe(bpy)3]3+ [Co(edta)]2- 33middot104 29 -63[Co(edta)]- [Fe(pdta)]2 13middot101 30 -128[IrCl3]2- [Ru(CN)6]4- 66middot104 19 -88[Co(ox)3]3- [Ru(NH3)6]2+ 18middot10-1 45 -108[Fe(CN)6]3- [Co(phen)3]2+ 60middot106
Oxidant Reductant k (M-1 s-1) ∆Hne (kJ mol-1) ∆Sne (J K-1 mol-1)
Electron Transfer Reactions some more activation parameters for outer sphere reactions
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
Electron Transfer Reactions The electron transfer step
++++ +rarr+ 363
23
263
33 ])[Ru(NH]Co(phen)])[Ru(NH][Co(phen)
A B Alsquo Blsquo
overall electron transfer rate constant 15middot104 M-1 s-1
KOS = 025 M-1 small due to repulsion
CoN
N N
N
N
N
3+
Ru
H2O
H2O OH2
OH2
OH2
OH2
2+
Ru
H2O
H2O OH2
OH2
OH2
OH2
3+Co
N
N N
N
N
N
2+
A Co-N = 191 Aring Alsquo Co-N = 211 AringB Ru-N = 214 Aring Blsquo Ru-N = 211 Aring
kET
relaxation leads to energy dissipation Thermodynamics
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 ACIV-FS20-T2-96
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
electron transfer occurs after structural changes
which can be illustrated with a potential diagram
reaction profile
describes precursor p (x=0) and product (x=1)
e--transfer probability=1 where they intersect
2pp xG sdot= λ deg+minus= ΔGx)(1G 2
ss λ
2)G1(41G λλ deg∆+=∆ ne
∆G dependent on ∆Gdeg and λpλs
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 ACIV-FS20-T2-97
Electron Transfer Reactions The electron transfer step
electron transfer must obey the Franck-Condon principles
different possibilities
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
Electron Transfer Reactions The electron transfer step
The Marcus equation for outer sphere e--transfer reactions
A- + B A + B-os reaction
A- + A A + A-
B- + B B + B-
kAB
kAA
kBB
e--self exchange rates
The different states can be expressed as follows
GneAA = G ne(A) + G ne(A-) ndash Gdeg(A) ndash Gdeg(A-)
GneBB = G ne(B) + G ne(B-) ndash Gdeg(B) ndash Gdeg(B-)
GneAB = G ne(A-) + G ne(B) ndash Gdeg(B) ndash Gdeg(A-)
GdegAB = Gdeg(B-) + Gdeg(A) ndash Gdeg(B) ndash Gdeg(A-)
Kinetics
Thermodynamics
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
Electron Transfer Reactions The electron transfer step
with some algebra (up to you) and the principle of microscopic reversibility
we arrive at ∆GneAB = ∆G ne(AA) + ∆G ne(BB) + ∆Gdeg(AB)
if this is translated into rate constants with kij= Zijmiddotexp -∆Gneij
RTij=AB
kAB= (kAAmiddotkBBmiddotKABmiddotF)frac12 Marcus correlation F = Z2
AB
ZAAmiddotZBB
if the charges remain the same left and right F asymp 1
eg Fe3+ + Cr2+ Fe2+ + Cr3+
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
Electron Transfer Reactions The electron transfer step
electron self exchange rates∆ddeg kAA(M-1s-1 ∆ddeg kAA(M-1s-1
[Fe(OH2)6]3+2+ 013 11 [Co(phen)3]3+2+ 019 12
[Co(OH2)6]3+2+ 021 5 [Fe(phen)3]3+2+ 0 13middot107
[Cr(OH2)6]3+2+ 020 19middot10-5 [Co(bipy)3]3+2+ 019 57
[V(OH2)6]3+2+ 013 1middot10-2 [Ru(bipy)3]3+2+ 0 4middot108
[Co(en)3]3+2+ 021 77middot10-5 [Fe(bipy)3]3+2+ 0 3middot108
[Co(NH3)6]3+2+ 022 2middot10-8 [Cr(bipy)3]3+2+ 01 109
[Ru(en)3]3+2+ 005 31middot104 [Fe(CN)6]3-4- 003 2middot104
kexp(M-1s-1) kcalc(M-1s-1)[Co(OH2)6]3+ + [Fe(OH2)6]2+ 50 4
[Co(OH2)6]3+ + [V(OH2)6]2+ 9middot105 2middot106
[Co(OH2)6]3+ + [Cr(OH2)6]2+ 13middot104 7middot103
[V(OH2)6]3+ + [Cr(OH2)6]2+ 02 2middot10-5
[Fe(OH2)6]3+ + [Cr(OH2)6]2+ 57middot102 68
[Fe(OH2)6]3+ + [V(OH2)6]2+ 18middot104 3middot104
[Fe(OH2)6]3+ + [Ru(OH2)6]2+ 23middot103 1middot103
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
pm large structural and electronic changes decelerates (emdashtransfer) reactions
[Co(NH3)6]3+ (ls) [Co(NH3)6]2+ (hs) [Co(NH3)6]2+ (hs) [Co(NH3)6]3+ (ls)
ket = 2middot10-8
ket = 6middot103
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
ket = 6middot10-3
[Ru(NH3)6]3+ (ls) [Ru(NH3)6]2+ (ls) [Co(NH3)6]2+ (ls) [Co(NH3)6]3+ (ls)
t2g rarr t2g much faster than eg rarr eg
t2g orbitals are not pointing towards the ligands but in between
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
Electron Transfer Reactions The electron transfer step
influence of d-electron configuration
kexp(M-1s-1) kcalc(M-1s-1)[Fe(H2O)6]2+ + [Fe(H2O)6]3+ 4 (t2g)4(eg)2 rarr (t2g)3(eg)2
[Fe(phen)3]2+ + [Fe(phen)3]3+ 3 middot 107 (t2g)6 rarr (t2g)5
[Ru(NH3)6]2+ + [Ru(NH3)6]3+ 82 middot 102 (t2g)6 rarr (t2g)5
[Ru(phen)3]2+ + [Ru(phen)3]3+ gt 107 (t2g)6 rarr (t2g)5
[Co(H2O)6]2+ + [Co(H2O)6]3+ 5 (t2g)5(eg) rarr (t2g)6
[Co(NH3)6]2+ + [Co(NH3)6]3+ 2middot10-8 (t2g)5(eg)2 rarr (t2g)6
[Co(en)3]2+ + [Co(en)3]3+ 14 sdot 10-4 (t2g)5(eg)2 rarr (t2g)6
[Co(phen)3]2+ + [Co(phen)3]3+ 11 (t2g)5(eg)2 rarr (t2g)6
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
mittel
weich
hart
Reduktionsmittel Oxidationsmittel Bruumlcke = X k2 (25 degC) [M-1s-1]
Cr2+ Co(NH3)5Xn+ CH3COO 18 bull 10-1
Cr2+ Co(NH3)5Xn+ ndashNCS 19 bull 101
Cr2+ Co(NH3)5Xn+ ndashCN 36 bull 101
Cr2+ Co(NH3)5Xn+ ndashF 25 bull 105
Cr2+ Co(NH3)5Xn+ ndashN3 30 bull 105
Cr2+ Co(NH3)5Xn+ ndashCl 60 bull 105
Cr2+ Co(NH3)5Xn+ ndashBr 14 bull 106
Cr2+ Co(NH3)5Xn+ ndashI 34 bull 106
Cr2+ Co(NH3)5Xn+ ndashPO4 48 bull 109
Cr2+ Cr(NH3)5X2+ ndashF 27 bull 10-4
Cr2+ Cr(NH3)5X2+ ndashCl 51 bull 10-2
Cr2+ Cr(NH3)5X2+ ndashBr 32 bull 10-1
Co(CN)53- Co(NH3)5Xn+ ndashPO4 52 bull 102
Co(CN)53- Co(NH3)5Xn+ ndashNCS 11 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashN3 16 bull 106
Co(CN)53- Co(NH3)5Xn+ ndashCl 50 bull 107
Co(CN)53- Co(NH3)5Xn+ ndashBr 20 bull 109
Co(CN)53- Co(NH3)5Xn+ ndashI zu schnellFe2+ Co(NH3)5Xn+ ndashBr 73 bull 10-4
Fe2+ Co(NH3)5Xn+ ndashCl 14 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashNCS 30 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashF 66 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashN3 87 bull 10-3
Fe2+ Co(NH3)5Xn+ ndashSCN 12 bull 10-1
06052020 ACIV-FS20-T2-104
06052020 CHE323-FS20-T2-105
Electron Transfer Reactions Outer sphere electron transfer reactions
criteria for outer sphere reactions
Outer sphere electron transfer if no bridging ligands available
The electron transfer limits the rate of outer sphere processes
Outer sphere electron transfer follows very often saturation kinetics features
Rates corresponding to predictions from Marcus outer sphere et is indicated
Electron transfer from t2g to t2g typically much faster than eg to eg
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- 5 Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Linear Free Energy Relationships LFER
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
- Electron Transfer Reactions
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- Electron Transfer Reactions
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![On the Non-triviality of the · Nuc. Phys. B 200 [FS4] 281 (1982) Callaway (Rockefeller University) ... Slide 1 Author: Loga Created Date: 12/8/2008 8:37:43 AM ...](https://static.fdocument.org/doc/165x107/5acca1227f8b9ad13e8d10de/on-the-non-triviality-of-phys-b-200-fs4-281-1982-callaway-rockefeller-university.jpg)
