lig and substitution
TRANSCRIPT
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Ligand substitution reactions:
dissociativeExample:
Factors influencing ease of dissociation:
1erow < 2erow > 3erow
d8-ML5> d10-ML4> d
6-ML6
stable ligands (CO, olefins, Cl-) dissociate easily
(as opposed to e.g. CH3, Cp).
LnM CO L
nM L
nM L'+ CO
L'
18 e 16 e 18 e
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Dissociative substitution in ML6
16-eML5complexes are usually fluxional;
the reaction proceeds with partial inversion,
partial retention of stereochemistry.
18-e
oct
16-e
SP distortedTBP
or
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Example:
Sometimes the solvent is involved:
Ligand substitution reactions:
associative
16 e 18 e
L'Ln-1M L'LnM LnM L'
- L
16 e
L2MCl2 L2M(Cl)(Br)
L2M(Cl)(H2O)+
Br-
- Cl-
H2O - Cl- Br-
- H2O (cis-platin !)
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Ligand rearrangement
Several ligands can switch between n-eand
(n-2)-esituations, thus enabling associative
reactions:
M N
O
M N O
3-e 1-e
M M
3-e5-eM
CO
RM
O
R
(1+2)-e 1-e
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Ligand substitution reactions:
redox-induced
For 17-e19-eusually fast equilibrium:
Reduction promotes dissociative substitution.
Oxidation promotes associative substitution.
In favourable cases, the product
oxidizes/reduces the starting material
redox catalysis.
LnM L'L
nM + L'
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Initiation by added reductant.
Sometimes, radical abstraction produces a 17-e species
(see C103).
CO
L
Fe(CO)4L
Fe(CO)4L
Fe(CO)4
Fe(CO)5
Fe(CO)5
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Ligand substitution reactions:
photochemicalVisible light can excite an electron from an M-L
bonding orbital to an M-L antibonding orbital (LF
transition). This results in fast ligand dissociation.
Requirement: the complex must absorb,
so it must have a colour.
d
d
h
M(CO)6
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Some ligands have a low-lying * orbital and
undergo MLCT excitation. This leads to easyassociative substitution.
The excited state is formally (n-1)-e !
M-M bonds dissociate easily on irradiation
(n-1)-eassociative substitution
d
d
h
M(CO)4(phen)
*
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Ligand activation:
electrophilic and nucleophilic attack Electron-rich metal fragment:
ligands activated for electrophilic attack.
N4: strong -donor
NN
N
RhS
N
++
S
H+N
N
N
Rh
N
+
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Ligand activation:
electrophilic and nucleophilic attack Electron-poor metal fragment:
ligands activated for nucleophilic attack.
-
Cr
OCOC
CO
Bu
H
Li+
Cr
OCOC
CO
BuLi
CO: strong -acceptor
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Electrophilic attack on ligand
Hapticity may increase or decrease.
Formal oxidation state of metal may
increase.
H+MI
+
MI
H+M(0) MII
+
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Electrophilic addition
Electrophilic abstraction
Alkyl exchange also starts with electrophilic attack.
O
Fe(CO)3
OEt+
Fe(CO)3
Et3O+
Cp2Zr
Me
Me
B(C6F5)3Cp2Zr
Me
+
MeB(C6F5)3-
16 e 14 e
+
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Competition: attack of electrophile on metal
(may be followed by shift to ligand)
Fe
+
Fe H
Ni
H+
H+
Ni
+
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Electrophilic attack on metal
Can be the start of oxidative addition
Key reaction in the Monsanto acetic acid
process:
I2(CO)
2Rh Me I I
2(CO)
2RhMe + I- I
3(CO)
2RhMe
MeOH + CO MeCOOHHI"Rh"
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MeOH
MeI
HI
H2O
MeCOOH
MeCOI
Rh(CO)2I2
Rh(CO)2I3Me Rh(CO)2I3(COMe)
CO
oxidative
addition
reductive
elimination
insertion
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Nucleophilic attack on ligand
Nucleophile "substitutes" metal
hapticity usually decreases
Oxidation state mostly unchanged
Competition: nucleophilic attack on metal
usually leads to substitution
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Nucleophilic abstraction
NaH
CrOCOC
CO
Na+
-
CrOCOC
CO
Cp2WH2BuLi Cp2WH Li
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Nucleophilic addition
Key reaction of the Wacker process:
(H2O)Cl2Pd + OH- (H2O)Cl2Pd
OH
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C2H4 + O2PdCl2, H2O
CH3CHO
CuCl2
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H2O Pd Cl
Cl
H2O PdH
ClOH
H2O PdH
Cl O
Cl Pd ClCl
Cl 2--
Cl Pd Cl
Cl
H2O PdCl
OH
H2O PdCl
OH-H elim
- 2 e(CuCl2CuCl)
C2H4 H2O
H2O PdCl
ClOH
-
ins
Cl-
CH3CHO
H2O Pd ClH
Cl-
Pd(0)+ H++
H2O + 2 Cl-
OH-
- Cl-
-H elim
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How can you distinguish between internaland
externalattack of OH-?
Use trans-CHD=CHD and trap the intermediate
Pd-C-C-OH with CO:
H2O PdCl
Cl
H2O Pd
OH
Cl-
OH- H2O PdCl
Cl
OH-
-
H2O Pd
O
Cl
H
ins
OH- - Cl-??
H2O PdCl
ClOH
-
PdCO
OH
O
O
CO
- Cl-
Pd
O
HO
ins
nuclattack
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OH-
O
O
D
D
Pd
HO
D
D
Pd
HO
D
D
Pd
D
D
O
O
D
D
PdD
HO
D
Pd
OHD
D
CO
CO
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Could acetaldehyde be formed directly as
vinyl alcohol ?Perform reaction in D2O:
diss-H elim OPd
D
ins
CH3
OH
PdOD
D2OCH2D
OH
diss
ODOD
Pd
H