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...

Page 1: Deduction of Mechanismsf0b7d1a0-e882-4e69-9f9...∆S ≠ =4.58(logA −13.2) 06.05.2020 CHE323-FS20T2--2 Deduction of Mechanisms 4.2 Transition State Theory Arrhenius:

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

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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

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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

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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

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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

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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

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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

=

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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

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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

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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

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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

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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+

+

+

+==

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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

==

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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

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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

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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

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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

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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

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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+ --

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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)

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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

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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

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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]-

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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

EMBED WordPicture8

_1073908171unknown

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

  • Foliennummer 1
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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
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  • Electron Transfer Reactions
  • Electron Transfer Reactions
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  • Foliennummer 104
  • Electron Transfer Reactions

ReduktionsmittelOxidationsmittelBruumlcke = X k2 (25 degC) [M-1s-1]

Cr2+Co(NH3)5Xn+CH3COO18 bull 10-1

Cr2+Co(NH3)5Xn+ndashNCS19 bull 101

Cr2+Co(NH3)5Xn+ndashCN 36 bull 101

Cr2+Co(NH3)5Xn+ndashF25 bull 105

Cr2+Co(NH3)5Xn+ndashN330 bull 105

Cr2+Co(NH3)5Xn+ndashCl60 bull 105

Cr2+Co(NH3)5Xn+ndashBr14 bull 106

Cr2+Co(NH3)5Xn+ndashI34 bull 106

Cr2+Co(NH3)5Xn+ndashPO448 bull 109

Cr2+Cr(NH3)5X2+ndashF27 bull 10-4

Cr2+Cr(NH3)5X2+ndashCl51 bull 10-2

Cr2+Cr(NH3)5X2+ndashBr32 bull 10-1

Co(CN)53-Co(NH3)5Xn+ndashPO452 bull 102

Co(CN)53-Co(NH3)5Xn+ndashNCS11 bull 106

Co(CN)53-Co(NH3)5Xn+ndashN316 bull 106

Co(CN)53-Co(NH3)5Xn+ndashCl50 bull 107

Co(CN)53-Co(NH3)5Xn+ndashBr20 bull 109

Co(CN)53-Co(NH3)5Xn+ndashIzu schnell

Fe2+Co(NH3)5Xn+ndashBr73 bull 10-4

Fe2+Co(NH3)5Xn+ndashCl14 bull 10-3

Fe2+Co(NH3)5Xn+ndashNCS30 bull 10-3

Fe2+Co(NH3)5Xn+ndashF66 bull 10-3

Fe2+Co(NH3)5Xn+ndashN387 bull 10-3

Fe2+Co(NH3)5Xn+ndashSCN12 bull 10-1

Page 2: Deduction of Mechanismsf0b7d1a0-e882-4e69-9f9...∆S ≠ =4.58(logA −13.2) 06.05.2020 CHE323-FS20T2--2 Deduction of Mechanisms 4.2 Transition State Theory Arrhenius:

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

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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
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  • Electron Transfer Reactions

ReduktionsmittelOxidationsmittelBruumlcke = X k2 (25 degC) [M-1s-1]

Cr2+Co(NH3)5Xn+CH3COO18 bull 10-1

Cr2+Co(NH3)5Xn+ndashNCS19 bull 101

Cr2+Co(NH3)5Xn+ndashCN 36 bull 101

Cr2+Co(NH3)5Xn+ndashF25 bull 105

Cr2+Co(NH3)5Xn+ndashN330 bull 105

Cr2+Co(NH3)5Xn+ndashCl60 bull 105

Cr2+Co(NH3)5Xn+ndashBr14 bull 106

Cr2+Co(NH3)5Xn+ndashI34 bull 106

Cr2+Co(NH3)5Xn+ndashPO448 bull 109

Cr2+Cr(NH3)5X2+ndashF27 bull 10-4

Cr2+Cr(NH3)5X2+ndashCl51 bull 10-2

Cr2+Cr(NH3)5X2+ndashBr32 bull 10-1

Co(CN)53-Co(NH3)5Xn+ndashPO452 bull 102

Co(CN)53-Co(NH3)5Xn+ndashNCS11 bull 106

Co(CN)53-Co(NH3)5Xn+ndashN316 bull 106

Co(CN)53-Co(NH3)5Xn+ndashCl50 bull 107

Co(CN)53-Co(NH3)5Xn+ndashBr20 bull 109

Co(CN)53-Co(NH3)5Xn+ndashIzu schnell

Fe2+Co(NH3)5Xn+ndashBr73 bull 10-4

Fe2+Co(NH3)5Xn+ndashCl14 bull 10-3

Fe2+Co(NH3)5Xn+ndashNCS30 bull 10-3

Fe2+Co(NH3)5Xn+ndashF66 bull 10-3

Fe2+Co(NH3)5Xn+ndashN387 bull 10-3

Fe2+Co(NH3)5Xn+ndashSCN12 bull 10-1

Page 3: Deduction of Mechanismsf0b7d1a0-e882-4e69-9f9...∆S ≠ =4.58(logA −13.2) 06.05.2020 CHE323-FS20T2--2 Deduction of Mechanisms 4.2 Transition State Theory Arrhenius:

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

EMBED WordPicture8

_1073908171unknown

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

  • Foliennummer 1
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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
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  • Electron Transfer Reactions
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  • Foliennummer 104
  • Electron Transfer Reactions

ReduktionsmittelOxidationsmittelBruumlcke = X k2 (25 degC) [M-1s-1]

Cr2+Co(NH3)5Xn+CH3COO18 bull 10-1

Cr2+Co(NH3)5Xn+ndashNCS19 bull 101

Cr2+Co(NH3)5Xn+ndashCN 36 bull 101

Cr2+Co(NH3)5Xn+ndashF25 bull 105

Cr2+Co(NH3)5Xn+ndashN330 bull 105

Cr2+Co(NH3)5Xn+ndashCl60 bull 105

Cr2+Co(NH3)5Xn+ndashBr14 bull 106

Cr2+Co(NH3)5Xn+ndashI34 bull 106

Cr2+Co(NH3)5Xn+ndashPO448 bull 109

Cr2+Cr(NH3)5X2+ndashF27 bull 10-4

Cr2+Cr(NH3)5X2+ndashCl51 bull 10-2

Cr2+Cr(NH3)5X2+ndashBr32 bull 10-1

Co(CN)53-Co(NH3)5Xn+ndashPO452 bull 102

Co(CN)53-Co(NH3)5Xn+ndashNCS11 bull 106

Co(CN)53-Co(NH3)5Xn+ndashN316 bull 106

Co(CN)53-Co(NH3)5Xn+ndashCl50 bull 107

Co(CN)53-Co(NH3)5Xn+ndashBr20 bull 109

Co(CN)53-Co(NH3)5Xn+ndashIzu schnell

Fe2+Co(NH3)5Xn+ndashBr73 bull 10-4

Fe2+Co(NH3)5Xn+ndashCl14 bull 10-3

Fe2+Co(NH3)5Xn+ndashNCS30 bull 10-3

Fe2+Co(NH3)5Xn+ndashF66 bull 10-3

Fe2+Co(NH3)5Xn+ndashN387 bull 10-3

Fe2+Co(NH3)5Xn+ndashSCN12 bull 10-1

Page 4: Deduction of Mechanismsf0b7d1a0-e882-4e69-9f9...∆S ≠ =4.58(logA −13.2) 06.05.2020 CHE323-FS20T2--2 Deduction of Mechanisms 4.2 Transition State Theory Arrhenius:

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

EMBED WordPicture8

_1073908171unknown

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

  • Foliennummer 1
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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
  • Foliennummer 104
  • Electron Transfer Reactions

ReduktionsmittelOxidationsmittelBruumlcke = X k2 (25 degC) [M-1s-1]

Cr2+Co(NH3)5Xn+CH3COO18 bull 10-1

Cr2+Co(NH3)5Xn+ndashNCS19 bull 101

Cr2+Co(NH3)5Xn+ndashCN 36 bull 101

Cr2+Co(NH3)5Xn+ndashF25 bull 105

Cr2+Co(NH3)5Xn+ndashN330 bull 105

Cr2+Co(NH3)5Xn+ndashCl60 bull 105

Cr2+Co(NH3)5Xn+ndashBr14 bull 106

Cr2+Co(NH3)5Xn+ndashI34 bull 106

Cr2+Co(NH3)5Xn+ndashPO448 bull 109

Cr2+Cr(NH3)5X2+ndashF27 bull 10-4

Cr2+Cr(NH3)5X2+ndashCl51 bull 10-2

Cr2+Cr(NH3)5X2+ndashBr32 bull 10-1

Co(CN)53-Co(NH3)5Xn+ndashPO452 bull 102

Co(CN)53-Co(NH3)5Xn+ndashNCS11 bull 106

Co(CN)53-Co(NH3)5Xn+ndashN316 bull 106

Co(CN)53-Co(NH3)5Xn+ndashCl50 bull 107

Co(CN)53-Co(NH3)5Xn+ndashBr20 bull 109

Co(CN)53-Co(NH3)5Xn+ndashIzu schnell

Fe2+Co(NH3)5Xn+ndashBr73 bull 10-4

Fe2+Co(NH3)5Xn+ndashCl14 bull 10-3

Fe2+Co(NH3)5Xn+ndashNCS30 bull 10-3

Fe2+Co(NH3)5Xn+ndashF66 bull 10-3

Fe2+Co(NH3)5Xn+ndashN387 bull 10-3

Fe2+Co(NH3)5Xn+ndashSCN12 bull 10-1

Page 5: Deduction of Mechanismsf0b7d1a0-e882-4e69-9f9...∆S ≠ =4.58(logA −13.2) 06.05.2020 CHE323-FS20T2--2 Deduction of Mechanisms 4.2 Transition State Theory Arrhenius:

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

EMBED WordPicture8

_1073908171unknown

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
  • Foliennummer 104
  • Electron Transfer Reactions

ReduktionsmittelOxidationsmittelBruumlcke = X k2 (25 degC) [M-1s-1]

Cr2+Co(NH3)5Xn+CH3COO18 bull 10-1

Cr2+Co(NH3)5Xn+ndashNCS19 bull 101

Cr2+Co(NH3)5Xn+ndashCN 36 bull 101

Cr2+Co(NH3)5Xn+ndashF25 bull 105

Cr2+Co(NH3)5Xn+ndashN330 bull 105

Cr2+Co(NH3)5Xn+ndashCl60 bull 105

Cr2+Co(NH3)5Xn+ndashBr14 bull 106

Cr2+Co(NH3)5Xn+ndashI34 bull 106

Cr2+Co(NH3)5Xn+ndashPO448 bull 109

Cr2+Cr(NH3)5X2+ndashF27 bull 10-4

Cr2+Cr(NH3)5X2+ndashCl51 bull 10-2

Cr2+Cr(NH3)5X2+ndashBr32 bull 10-1

Co(CN)53-Co(NH3)5Xn+ndashPO452 bull 102

Co(CN)53-Co(NH3)5Xn+ndashNCS11 bull 106

Co(CN)53-Co(NH3)5Xn+ndashN316 bull 106

Co(CN)53-Co(NH3)5Xn+ndashCl50 bull 107

Co(CN)53-Co(NH3)5Xn+ndashBr20 bull 109

Co(CN)53-Co(NH3)5Xn+ndashIzu schnell

Fe2+Co(NH3)5Xn+ndashBr73 bull 10-4

Fe2+Co(NH3)5Xn+ndashCl14 bull 10-3

Fe2+Co(NH3)5Xn+ndashNCS30 bull 10-3

Fe2+Co(NH3)5Xn+ndashF66 bull 10-3

Fe2+Co(NH3)5Xn+ndashN387 bull 10-3

Fe2+Co(NH3)5Xn+ndashSCN12 bull 10-1

Page 6: Deduction of Mechanismsf0b7d1a0-e882-4e69-9f9...∆S ≠ =4.58(logA −13.2) 06.05.2020 CHE323-FS20T2--2 Deduction of Mechanisms 4.2 Transition State Theory Arrhenius:

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

EMBED WordPicture8

_1073908171unknown

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

  • Foliennummer 1
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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
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  • Foliennummer 104
  • Electron Transfer Reactions

ReduktionsmittelOxidationsmittelBruumlcke = X k2 (25 degC) [M-1s-1]

Cr2+Co(NH3)5Xn+CH3COO18 bull 10-1

Cr2+Co(NH3)5Xn+ndashNCS19 bull 101

Cr2+Co(NH3)5Xn+ndashCN 36 bull 101

Cr2+Co(NH3)5Xn+ndashF25 bull 105

Cr2+Co(NH3)5Xn+ndashN330 bull 105

Cr2+Co(NH3)5Xn+ndashCl60 bull 105

Cr2+Co(NH3)5Xn+ndashBr14 bull 106

Cr2+Co(NH3)5Xn+ndashI34 bull 106

Cr2+Co(NH3)5Xn+ndashPO448 bull 109

Cr2+Cr(NH3)5X2+ndashF27 bull 10-4

Cr2+Cr(NH3)5X2+ndashCl51 bull 10-2

Cr2+Cr(NH3)5X2+ndashBr32 bull 10-1

Co(CN)53-Co(NH3)5Xn+ndashPO452 bull 102

Co(CN)53-Co(NH3)5Xn+ndashNCS11 bull 106

Co(CN)53-Co(NH3)5Xn+ndashN316 bull 106

Co(CN)53-Co(NH3)5Xn+ndashCl50 bull 107

Co(CN)53-Co(NH3)5Xn+ndashBr20 bull 109

Co(CN)53-Co(NH3)5Xn+ndashIzu schnell

Fe2+Co(NH3)5Xn+ndashBr73 bull 10-4

Fe2+Co(NH3)5Xn+ndashCl14 bull 10-3

Fe2+Co(NH3)5Xn+ndashNCS30 bull 10-3

Fe2+Co(NH3)5Xn+ndashF66 bull 10-3

Fe2+Co(NH3)5Xn+ndashN387 bull 10-3

Fe2+Co(NH3)5Xn+ndashSCN12 bull 10-1

Page 7: Deduction of Mechanismsf0b7d1a0-e882-4e69-9f9...∆S ≠ =4.58(logA −13.2) 06.05.2020 CHE323-FS20T2--2 Deduction of Mechanisms 4.2 Transition State Theory Arrhenius:

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

EMBED WordPicture8

_1073908171unknown

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
  • Foliennummer 104
  • Electron Transfer Reactions

ReduktionsmittelOxidationsmittelBruumlcke = X k2 (25 degC) [M-1s-1]

Cr2+Co(NH3)5Xn+CH3COO18 bull 10-1

Cr2+Co(NH3)5Xn+ndashNCS19 bull 101

Cr2+Co(NH3)5Xn+ndashCN 36 bull 101

Cr2+Co(NH3)5Xn+ndashF25 bull 105

Cr2+Co(NH3)5Xn+ndashN330 bull 105

Cr2+Co(NH3)5Xn+ndashCl60 bull 105

Cr2+Co(NH3)5Xn+ndashBr14 bull 106

Cr2+Co(NH3)5Xn+ndashI34 bull 106

Cr2+Co(NH3)5Xn+ndashPO448 bull 109

Cr2+Cr(NH3)5X2+ndashF27 bull 10-4

Cr2+Cr(NH3)5X2+ndashCl51 bull 10-2

Cr2+Cr(NH3)5X2+ndashBr32 bull 10-1

Co(CN)53-Co(NH3)5Xn+ndashPO452 bull 102

Co(CN)53-Co(NH3)5Xn+ndashNCS11 bull 106

Co(CN)53-Co(NH3)5Xn+ndashN316 bull 106

Co(CN)53-Co(NH3)5Xn+ndashCl50 bull 107

Co(CN)53-Co(NH3)5Xn+ndashBr20 bull 109

Co(CN)53-Co(NH3)5Xn+ndashIzu schnell

Fe2+Co(NH3)5Xn+ndashBr73 bull 10-4

Fe2+Co(NH3)5Xn+ndashCl14 bull 10-3

Fe2+Co(NH3)5Xn+ndashNCS30 bull 10-3

Fe2+Co(NH3)5Xn+ndashF66 bull 10-3

Fe2+Co(NH3)5Xn+ndashN387 bull 10-3

Fe2+Co(NH3)5Xn+ndashSCN12 bull 10-1

Page 8: Deduction of Mechanismsf0b7d1a0-e882-4e69-9f9...∆S ≠ =4.58(logA −13.2) 06.05.2020 CHE323-FS20T2--2 Deduction of Mechanisms 4.2 Transition State Theory Arrhenius:

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

EMBED WordPicture8

_1073908171unknown

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

  • Foliennummer 1
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  • 5 Linear Free Energy Relationships LFER
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  • 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
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  • Linear Free Energy Relationships LFER
  • Linear Free Energy Relationships LFER
  • Linear Free Energy Relationships LFER
  • Electron Transfer Reactions
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  • Electron Transfer Reactions

ReduktionsmittelOxidationsmittelBruumlcke = X k2 (25 degC) [M-1s-1]

Cr2+Co(NH3)5Xn+CH3COO18 bull 10-1

Cr2+Co(NH3)5Xn+ndashNCS19 bull 101

Cr2+Co(NH3)5Xn+ndashCN 36 bull 101

Cr2+Co(NH3)5Xn+ndashF25 bull 105

Cr2+Co(NH3)5Xn+ndashN330 bull 105

Cr2+Co(NH3)5Xn+ndashCl60 bull 105

Cr2+Co(NH3)5Xn+ndashBr14 bull 106

Cr2+Co(NH3)5Xn+ndashI34 bull 106

Cr2+Co(NH3)5Xn+ndashPO448 bull 109

Cr2+Cr(NH3)5X2+ndashF27 bull 10-4

Cr2+Cr(NH3)5X2+ndashCl51 bull 10-2

Cr2+Cr(NH3)5X2+ndashBr32 bull 10-1

Co(CN)53-Co(NH3)5Xn+ndashPO452 bull 102

Co(CN)53-Co(NH3)5Xn+ndashNCS11 bull 106

Co(CN)53-Co(NH3)5Xn+ndashN316 bull 106

Co(CN)53-Co(NH3)5Xn+ndashCl50 bull 107

Co(CN)53-Co(NH3)5Xn+ndashBr20 bull 109

Co(CN)53-Co(NH3)5Xn+ndashIzu schnell

Fe2+Co(NH3)5Xn+ndashBr73 bull 10-4

Fe2+Co(NH3)5Xn+ndashCl14 bull 10-3

Fe2+Co(NH3)5Xn+ndashNCS30 bull 10-3

Fe2+Co(NH3)5Xn+ndashF66 bull 10-3

Fe2+Co(NH3)5Xn+ndashN387 bull 10-3

Fe2+Co(NH3)5Xn+ndashSCN12 bull 10-1

Page 9: Deduction of Mechanismsf0b7d1a0-e882-4e69-9f9...∆S ≠ =4.58(logA −13.2) 06.05.2020 CHE323-FS20T2--2 Deduction of Mechanisms 4.2 Transition State Theory Arrhenius:

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

EMBED WordPicture8

_1073908171unknown

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

  • Foliennummer 1
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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
  • Foliennummer 104
  • Electron Transfer Reactions

ReduktionsmittelOxidationsmittelBruumlcke = X k2 (25 degC) [M-1s-1]

Cr2+Co(NH3)5Xn+CH3COO18 bull 10-1

Cr2+Co(NH3)5Xn+ndashNCS19 bull 101

Cr2+Co(NH3)5Xn+ndashCN 36 bull 101

Cr2+Co(NH3)5Xn+ndashF25 bull 105

Cr2+Co(NH3)5Xn+ndashN330 bull 105

Cr2+Co(NH3)5Xn+ndashCl60 bull 105

Cr2+Co(NH3)5Xn+ndashBr14 bull 106

Cr2+Co(NH3)5Xn+ndashI34 bull 106

Cr2+Co(NH3)5Xn+ndashPO448 bull 109

Cr2+Cr(NH3)5X2+ndashF27 bull 10-4

Cr2+Cr(NH3)5X2+ndashCl51 bull 10-2

Cr2+Cr(NH3)5X2+ndashBr32 bull 10-1

Co(CN)53-Co(NH3)5Xn+ndashPO452 bull 102

Co(CN)53-Co(NH3)5Xn+ndashNCS11 bull 106

Co(CN)53-Co(NH3)5Xn+ndashN316 bull 106

Co(CN)53-Co(NH3)5Xn+ndashCl50 bull 107

Co(CN)53-Co(NH3)5Xn+ndashBr20 bull 109

Co(CN)53-Co(NH3)5Xn+ndashIzu schnell

Fe2+Co(NH3)5Xn+ndashBr73 bull 10-4

Fe2+Co(NH3)5Xn+ndashCl14 bull 10-3

Fe2+Co(NH3)5Xn+ndashNCS30 bull 10-3

Fe2+Co(NH3)5Xn+ndashF66 bull 10-3

Fe2+Co(NH3)5Xn+ndashN387 bull 10-3

Fe2+Co(NH3)5Xn+ndashSCN12 bull 10-1

Page 10: Deduction of Mechanismsf0b7d1a0-e882-4e69-9f9...∆S ≠ =4.58(logA −13.2) 06.05.2020 CHE323-FS20T2--2 Deduction of Mechanisms 4.2 Transition State Theory Arrhenius:

∆+∆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

EMBED WordPicture8

_1073908171unknown

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

  • Foliennummer 1
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  • 5 Linear Free Energy Relationships LFER
  • Linear Free Energy Relationships LFER
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  • 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
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  • Electron Transfer Reactions
  • Electron Transfer Reactions
  • Electron Transfer Reactions
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  • Foliennummer 104
  • Electron Transfer Reactions

ReduktionsmittelOxidationsmittelBruumlcke = X k2 (25 degC) [M-1s-1]

Cr2+Co(NH3)5Xn+CH3COO18 bull 10-1

Cr2+Co(NH3)5Xn+ndashNCS19 bull 101

Cr2+Co(NH3)5Xn+ndashCN 36 bull 101

Cr2+Co(NH3)5Xn+ndashF25 bull 105

Cr2+Co(NH3)5Xn+ndashN330 bull 105

Cr2+Co(NH3)5Xn+ndashCl60 bull 105

Cr2+Co(NH3)5Xn+ndashBr14 bull 106

Cr2+Co(NH3)5Xn+ndashI34 bull 106

Cr2+Co(NH3)5Xn+ndashPO448 bull 109

Cr2+Cr(NH3)5X2+ndashF27 bull 10-4

Cr2+Cr(NH3)5X2+ndashCl51 bull 10-2

Cr2+Cr(NH3)5X2+ndashBr32 bull 10-1

Co(CN)53-Co(NH3)5Xn+ndashPO452 bull 102

Co(CN)53-Co(NH3)5Xn+ndashNCS11 bull 106

Co(CN)53-Co(NH3)5Xn+ndashN316 bull 106

Co(CN)53-Co(NH3)5Xn+ndashCl50 bull 107

Co(CN)53-Co(NH3)5Xn+ndashBr20 bull 109

Co(CN)53-Co(NH3)5Xn+ndashIzu schnell

Fe2+Co(NH3)5Xn+ndashBr73 bull 10-4

Fe2+Co(NH3)5Xn+ndashCl14 bull 10-3

Fe2+Co(NH3)5Xn+ndashNCS30 bull 10-3

Fe2+Co(NH3)5Xn+ndashF66 bull 10-3

Fe2+Co(NH3)5Xn+ndashN387 bull 10-3

Fe2+Co(NH3)5Xn+ndashSCN12 bull 10-1

Page 11: Deduction of Mechanismsf0b7d1a0-e882-4e69-9f9...∆S ≠ =4.58(logA −13.2) 06.05.2020 CHE323-FS20T2--2 Deduction of Mechanisms 4.2 Transition State Theory Arrhenius:

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

EMBED WordPicture8

_1073908171unknown

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

  • Foliennummer 1
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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
  • Foliennummer 104
  • Electron Transfer Reactions

ReduktionsmittelOxidationsmittelBruumlcke = X k2 (25 degC) [M-1s-1]

Cr2+Co(NH3)5Xn+CH3COO18 bull 10-1

Cr2+Co(NH3)5Xn+ndashNCS19 bull 101

Cr2+Co(NH3)5Xn+ndashCN 36 bull 101

Cr2+Co(NH3)5Xn+ndashF25 bull 105

Cr2+Co(NH3)5Xn+ndashN330 bull 105

Cr2+Co(NH3)5Xn+ndashCl60 bull 105

Cr2+Co(NH3)5Xn+ndashBr14 bull 106

Cr2+Co(NH3)5Xn+ndashI34 bull 106

Cr2+Co(NH3)5Xn+ndashPO448 bull 109

Cr2+Cr(NH3)5X2+ndashF27 bull 10-4

Cr2+Cr(NH3)5X2+ndashCl51 bull 10-2

Cr2+Cr(NH3)5X2+ndashBr32 bull 10-1

Co(CN)53-Co(NH3)5Xn+ndashPO452 bull 102

Co(CN)53-Co(NH3)5Xn+ndashNCS11 bull 106

Co(CN)53-Co(NH3)5Xn+ndashN316 bull 106

Co(CN)53-Co(NH3)5Xn+ndashCl50 bull 107

Co(CN)53-Co(NH3)5Xn+ndashBr20 bull 109

Co(CN)53-Co(NH3)5Xn+ndashIzu schnell

Fe2+Co(NH3)5Xn+ndashBr73 bull 10-4

Fe2+Co(NH3)5Xn+ndashCl14 bull 10-3

Fe2+Co(NH3)5Xn+ndashNCS30 bull 10-3

Fe2+Co(NH3)5Xn+ndashF66 bull 10-3

Fe2+Co(NH3)5Xn+ndashN387 bull 10-3

Fe2+Co(NH3)5Xn+ndashSCN12 bull 10-1

Page 12: Deduction of Mechanismsf0b7d1a0-e882-4e69-9f9...∆S ≠ =4.58(logA −13.2) 06.05.2020 CHE323-FS20T2--2 Deduction of Mechanisms 4.2 Transition State Theory Arrhenius:

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

EMBED WordPicture8

_1073908171unknown

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

  • Foliennummer 1
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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
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  • 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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  • Foliennummer 104
  • Electron Transfer Reactions

ReduktionsmittelOxidationsmittelBruumlcke = X k2 (25 degC) [M-1s-1]

Cr2+Co(NH3)5Xn+CH3COO18 bull 10-1

Cr2+Co(NH3)5Xn+ndashNCS19 bull 101

Cr2+Co(NH3)5Xn+ndashCN 36 bull 101

Cr2+Co(NH3)5Xn+ndashF25 bull 105

Cr2+Co(NH3)5Xn+ndashN330 bull 105

Cr2+Co(NH3)5Xn+ndashCl60 bull 105

Cr2+Co(NH3)5Xn+ndashBr14 bull 106

Cr2+Co(NH3)5Xn+ndashI34 bull 106

Cr2+Co(NH3)5Xn+ndashPO448 bull 109

Cr2+Cr(NH3)5X2+ndashF27 bull 10-4

Cr2+Cr(NH3)5X2+ndashCl51 bull 10-2

Cr2+Cr(NH3)5X2+ndashBr32 bull 10-1

Co(CN)53-Co(NH3)5Xn+ndashPO452 bull 102

Co(CN)53-Co(NH3)5Xn+ndashNCS11 bull 106

Co(CN)53-Co(NH3)5Xn+ndashN316 bull 106

Co(CN)53-Co(NH3)5Xn+ndashCl50 bull 107

Co(CN)53-Co(NH3)5Xn+ndashBr20 bull 109

Co(CN)53-Co(NH3)5Xn+ndashIzu schnell

Fe2+Co(NH3)5Xn+ndashBr73 bull 10-4

Fe2+Co(NH3)5Xn+ndashCl14 bull 10-3

Fe2+Co(NH3)5Xn+ndashNCS30 bull 10-3

Fe2+Co(NH3)5Xn+ndashF66 bull 10-3

Fe2+Co(NH3)5Xn+ndashN387 bull 10-3

Fe2+Co(NH3)5Xn+ndashSCN12 bull 10-1

Page 13: Deduction of Mechanismsf0b7d1a0-e882-4e69-9f9...∆S ≠ =4.58(logA −13.2) 06.05.2020 CHE323-FS20T2--2 Deduction of Mechanisms 4.2 Transition State Theory Arrhenius:

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

EMBED WordPicture8

_1073908171unknown

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
  • Foliennummer 104
  • Electron Transfer Reactions

ReduktionsmittelOxidationsmittelBruumlcke = X k2 (25 degC) [M-1s-1]

Cr2+Co(NH3)5Xn+CH3COO18 bull 10-1

Cr2+Co(NH3)5Xn+ndashNCS19 bull 101

Cr2+Co(NH3)5Xn+ndashCN 36 bull 101

Cr2+Co(NH3)5Xn+ndashF25 bull 105

Cr2+Co(NH3)5Xn+ndashN330 bull 105

Cr2+Co(NH3)5Xn+ndashCl60 bull 105

Cr2+Co(NH3)5Xn+ndashBr14 bull 106

Cr2+Co(NH3)5Xn+ndashI34 bull 106

Cr2+Co(NH3)5Xn+ndashPO448 bull 109

Cr2+Cr(NH3)5X2+ndashF27 bull 10-4

Cr2+Cr(NH3)5X2+ndashCl51 bull 10-2

Cr2+Cr(NH3)5X2+ndashBr32 bull 10-1

Co(CN)53-Co(NH3)5Xn+ndashPO452 bull 102

Co(CN)53-Co(NH3)5Xn+ndashNCS11 bull 106

Co(CN)53-Co(NH3)5Xn+ndashN316 bull 106

Co(CN)53-Co(NH3)5Xn+ndashCl50 bull 107

Co(CN)53-Co(NH3)5Xn+ndashBr20 bull 109

Co(CN)53-Co(NH3)5Xn+ndashIzu schnell

Fe2+Co(NH3)5Xn+ndashBr73 bull 10-4

Fe2+Co(NH3)5Xn+ndashCl14 bull 10-3

Fe2+Co(NH3)5Xn+ndashNCS30 bull 10-3

Fe2+Co(NH3)5Xn+ndashF66 bull 10-3

Fe2+Co(NH3)5Xn+ndashN387 bull 10-3

Fe2+Co(NH3)5Xn+ndashSCN12 bull 10-1

Page 14: Deduction of Mechanismsf0b7d1a0-e882-4e69-9f9...∆S ≠ =4.58(logA −13.2) 06.05.2020 CHE323-FS20T2--2 Deduction of Mechanisms 4.2 Transition State Theory Arrhenius:

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

EMBED WordPicture8

_1073908171unknown

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

  • Foliennummer 1
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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
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  • Foliennummer 104
  • Electron Transfer Reactions

ReduktionsmittelOxidationsmittelBruumlcke = X k2 (25 degC) [M-1s-1]

Cr2+Co(NH3)5Xn+CH3COO18 bull 10-1

Cr2+Co(NH3)5Xn+ndashNCS19 bull 101

Cr2+Co(NH3)5Xn+ndashCN 36 bull 101

Cr2+Co(NH3)5Xn+ndashF25 bull 105

Cr2+Co(NH3)5Xn+ndashN330 bull 105

Cr2+Co(NH3)5Xn+ndashCl60 bull 105

Cr2+Co(NH3)5Xn+ndashBr14 bull 106

Cr2+Co(NH3)5Xn+ndashI34 bull 106

Cr2+Co(NH3)5Xn+ndashPO448 bull 109

Cr2+Cr(NH3)5X2+ndashF27 bull 10-4

Cr2+Cr(NH3)5X2+ndashCl51 bull 10-2

Cr2+Cr(NH3)5X2+ndashBr32 bull 10-1

Co(CN)53-Co(NH3)5Xn+ndashPO452 bull 102

Co(CN)53-Co(NH3)5Xn+ndashNCS11 bull 106

Co(CN)53-Co(NH3)5Xn+ndashN316 bull 106

Co(CN)53-Co(NH3)5Xn+ndashCl50 bull 107

Co(CN)53-Co(NH3)5Xn+ndashBr20 bull 109

Co(CN)53-Co(NH3)5Xn+ndashIzu schnell

Fe2+Co(NH3)5Xn+ndashBr73 bull 10-4

Fe2+Co(NH3)5Xn+ndashCl14 bull 10-3

Fe2+Co(NH3)5Xn+ndashNCS30 bull 10-3

Fe2+Co(NH3)5Xn+ndashF66 bull 10-3

Fe2+Co(NH3)5Xn+ndashN387 bull 10-3

Fe2+Co(NH3)5Xn+ndashSCN12 bull 10-1

Page 15: Deduction of Mechanismsf0b7d1a0-e882-4e69-9f9...∆S ≠ =4.58(logA −13.2) 06.05.2020 CHE323-FS20T2--2 Deduction of Mechanisms 4.2 Transition State Theory Arrhenius:

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

EMBED WordPicture8

_1073908171unknown

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
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  • Electron Transfer Reactions
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  • Electron Transfer Reactions

ReduktionsmittelOxidationsmittelBruumlcke = X k2 (25 degC) [M-1s-1]

Cr2+Co(NH3)5Xn+CH3COO18 bull 10-1

Cr2+Co(NH3)5Xn+ndashNCS19 bull 101

Cr2+Co(NH3)5Xn+ndashCN 36 bull 101

Cr2+Co(NH3)5Xn+ndashF25 bull 105

Cr2+Co(NH3)5Xn+ndashN330 bull 105

Cr2+Co(NH3)5Xn+ndashCl60 bull 105

Cr2+Co(NH3)5Xn+ndashBr14 bull 106

Cr2+Co(NH3)5Xn+ndashI34 bull 106

Cr2+Co(NH3)5Xn+ndashPO448 bull 109

Cr2+Cr(NH3)5X2+ndashF27 bull 10-4

Cr2+Cr(NH3)5X2+ndashCl51 bull 10-2

Cr2+Cr(NH3)5X2+ndashBr32 bull 10-1

Co(CN)53-Co(NH3)5Xn+ndashPO452 bull 102

Co(CN)53-Co(NH3)5Xn+ndashNCS11 bull 106

Co(CN)53-Co(NH3)5Xn+ndashN316 bull 106

Co(CN)53-Co(NH3)5Xn+ndashCl50 bull 107

Co(CN)53-Co(NH3)5Xn+ndashBr20 bull 109

Co(CN)53-Co(NH3)5Xn+ndashIzu schnell

Fe2+Co(NH3)5Xn+ndashBr73 bull 10-4

Fe2+Co(NH3)5Xn+ndashCl14 bull 10-3

Fe2+Co(NH3)5Xn+ndashNCS30 bull 10-3

Fe2+Co(NH3)5Xn+ndashF66 bull 10-3

Fe2+Co(NH3)5Xn+ndashN387 bull 10-3

Fe2+Co(NH3)5Xn+ndashSCN12 bull 10-1

Page 16: Deduction of Mechanismsf0b7d1a0-e882-4e69-9f9...∆S ≠ =4.58(logA −13.2) 06.05.2020 CHE323-FS20T2--2 Deduction of Mechanisms 4.2 Transition State Theory Arrhenius:

][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-middotmiddotmiddotmiddotmid