2. Details of - Colorado State...
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Transcript of 2. Details of - Colorado State...
++
_+
2. Perpendicular
•••
2. Details of π-bonding.
a. Organic π-bond as a σ-donor
++
+
filled πvacant"dsp"
b. Organic π*-orbital as π-acceptor
+_
+
+
_
_
filled d vacant π* back donation / back bonding
Bonding of unsaturated organicligands involves a balance ofthese bonding modes -- whichdepopulates π-MO and populatesπ*
changes chemistry of organicligand
2 types of π-acceptors
1. Longitudinal
++
+_
_
++
+_
_
π bondfilled sp
vacantdsp
filled dπ*
_
_ ++
+
π bondσ bond
π
π*filled dvacantdsp
C O
:C O RN C: C C C C
M
M
M M
D. Ligand substitution processes.
1. General Considerations - ligand substitution reactions are central to all organometallic reactions.
+ L' + L
a. For catalysis, substrate must coordinate to metal for activation - usually by ligand substitution of substrate for L.
b. To adjust reactivity of coordinated substrate, exchange an innocent ligand.
e.g.:
2
+ Et3N
unreactive towardnucleophiles
+
reactive towardnucleophiles
c. To stabilize normally unstable species.
e.g.:
2unstable to air
requires HNO3for oxidation
+
requires aqua regiaHCl/HNO3
M L M L'
PdCl
ClPdPd
NEt3
Cl
Cl Cl
NEt3
NEt3
NiBr
NiBr
PPh3
PPh2Ph2P
NiP
P
Et3N Cl-
PPh3
Ph2
Ph2
Br
2. Classification
Substitution Reactions
2e- (L:) 1e- (radical, redox)
Associative (Sn2-like)
Dissociative (Sn1-like)
MLn + L' L'MLn
L + L'MLn-1requires vacantsite - e.g.coordinativeunsaturation
MLn MLn-1 + L
L'
L + L'MLn-1
usually coord. sat.
Associative (SRN2)
Diassociative (SRN1)
Oxidatively or reductivelydriven ligand exchange(catalyzed).
Radical chain ligandexchange - later.
3. Examples
a. 16 - electron compounds - d8, square planar, unsaturated complexes of Ni(II), Pd(II), Pt(II), Rh(I), Ir(I) - most extensively studied. Typical mechanism is 2e-, associative.
e.g.:
+ Y
Sn2 - apical attack sq. py. trig. bip.
+ X
sq. py.
LcPt
X
Lt Lc
PtLcLt
XLc
Y Lc
Pt
Pt
X
XLt
YLc
Lc Y
Lt Lc
PtLcLt
YLc
Rate = 2nd order
Rate depends on the metal e.g. Ni > Pd >> Pt Å 106 difference
∴ if reaction depends on ligand exchange, Pt-catalyzed process may be too slow to be useful.
Rate depends on the incoming ligand R3P > Py > NH3, Cl– > H2O > OH– Å 105 range.
Rate depends on leaving group NO3– > H2O > Cl– > Br– > I– >
N3– > SCN– > NO2
– > CN–.
Rate depends on ligand trans to the one being displaced (trans effect) R3Si– > H– Å CH3
– Å CN– Å olefins Å CO > PR3 Å NO2
– Å I– Å SCN_ > Br > Cl– > RNH2 Å NH3 > OH– > NO3–
Å H2O
∴ you can activate ligand exchanges by replacing innocent ligand trans to the one you want to replace.
Sn2 - like
NOTE Trans effect is a kinetic phenomenon.
Trans influence- weakening of the bond trans to it.
c. 18-electron systems - much slower than 16e systems, coordinatively saturated - mostly dissociative.
Ni(CO)4 Fe(CO)5 Cr(CO)6
e.g.
•• L•
Look at MO schemegains 1/2 M-L bondby associating!
a1σ
WHY?*a1σ*
e + b2
e + b2
(R3P)V(CO)6
Rate = 2nd order, SN2 like ligand trans to leaving group often hasprofound effect on rate of exchange.
b. 17-electron systems - very labile - originally thought to be dissociative to 15e- system, but really associative to 19e- system.
•Mn(CO)5 + R3P •Mn(CO)4PR3 + CO
e.g. •V(CO)6 + R3P •V(CO)5Pr3 + CO (M(O), d5) Rate ~ [V(CO)6] [R3P] and depends on basicity of R3P
slow Ni(CO)3 + CO L
fastLNi(CO)3
tetrahedrallabile
Rate α [Ni(CO)4] - 1st order
L
only 1 e– into anti
2e– into bonding MO
for Mn(CO)5•
d7
MnMn
Ni(CO)4
b1 b2
a1
e + b2
a1σ*
a1σ
L
e + b2
•
C4v yzx
•• LMn Mn
Can be accelerated by bulky ligands - steric strain release
PhH 25°+ L
In contrast Fe(CO)5, Cr(CO)6 not labile, and require heat or hν to give dissociation of CO for ligand substitution.
Apparent associative ligand exchanges of 18e- systems occur by "slippage".
L
M(I), d6, 18e-, sat. M(I), d6, 16e-, unsat. isolated
L
NiL
L
L
LNi L
L
L
Mn(CO)3Mn(CO)3 Mn(CO)3L
Mn(CO)2
L =
Cone <
KD
P(OEt)3
109°
<1010
P(Optolyl)3
128°
6 x 10-10
P(O-iPr)3
130°
2.7 x 0-5
P(O-otolyl)3
141°
4 x 10-2
PPh3
145°
No NiL4
KD
-CO
19e-
+ MeCN
++
+ MeCN
d. Catalyzed and promoted ligand exchanges.
1. Electron-transfer catalysis.
- e
E° = 0.19v
_• •Ph3P
fast
•
L
M(I), d6, sat., 18e-unreactive toward L
17e- E°=0.52voxidizes
S.M.
for catalysis
L
chemical oxidants arealso effective
Reductive systems also known.
Fe(CO)5 + [Ph2CO] •– –•dissociative
•–
17e-
L
[LFe(CO)4] –•
M(0), d8, sat. 18e-
LFe(CO)4
strongreducingagent
Mn(CO)2Mn(MeCN) Mn(CO)2
Mn(CO)2
(CH3CN) (CO)2
[Fe(CO)5]-CO
[Fe(CO)4]
2. Radical chain
M(I), d6, 18e- M(0), d7, 17e-
•Re(CO)5 + InH
•Re(CO)5 + L LRe(CO)4• + CO (associative as above)
LRe(CO)4• + HRe(CO)5 LRe(CO)4H + Re(CO)5•
3. Other assisted ligand substitutions
Most important is R3NO replacements of CO on metal carbonyls.
Fe(CO)5 + R3N+_O_ Nuc.
attack
_ +
M(0), d8, 18e-, sat.not labile
R3N + LFe(CO)4L [(CO)4FeNR3 + CO2]
L
Note! Bu3PO catalyzes this kind of CO replacement.
Fe(CO)5 + R3PO+_
strong P-O bond
Fe(CO)4 + CO + R3POLFe(CO)4
(CO)4Fe CO
O NR3
O PR3CO
(CO)4Fe
HRe(CO)5In
