Metal-carbene complexes contain transition metals bonded to carbene ligands. There are two main types: Shrock and Fischer carbene complexes. Shrock carbenes use early transition metals and form via α-elimination, with the carbene carbon being nucleophilic. Fischer carbenes use late transition metals and form by nucleophilic attack, with the carbene carbon being electrophilic. Applications include alkene metathesis catalysis and the Fischer-Tropsch process for hydrocarbon synthesis.

1. Metal-Carbene Complexes
1) Introduction
2) Types of metal Carbene complexes
1) Shrock Carbenes
1) Synthesis
2) Reactivity
2) Fischer Carbenes
1) Synthesis
2) Reactivity
3) N-heterocyclic carbenes
3) Applications

2. Metal-Carbene Complexes
Metal carbene complex is an organometallic compound with
divalent organic ligand
Carbene complexes for almost all transition metals have been
reported.
The term carbene is used formally as many are not derived from
carbenes and almost none exhibit the reactivity of carbenes.

3. Metal-carbon multiple bonds
• Many transition metals form not only M-C single bonds but also
M=C and (more rare) even M≡C bonds.
• Complexes containing an M=C bond are called
carbene complexes
– The ligand without the metal would be a free carbene
• Complexes with M≡C bonds are called carbyne complexes
• Both types of complexes tend to be reactive,
and useful in catalysis
• Fischer-type and Schrock-type carbene (and carbyne)
complexes can be distinguished

4. Schrock-type carbene complexes
• "Standard" M=C bond:
normal s and p bonds,
both polarized as in Md+-Cd-
• Carbene not particularly stabilized,
has a triplet ground state
• Early/middle transition metal,
high oxidation state
– count carbene as 2-
• Carbene carbon is nucleophilic
• hydrogen and alkyl substituents on carbenoid carbon
• Pi acceptor ligands
• Prototype:
– formed by "a-elimination"
M s
p
M
Ta
5
Ta

5. Synthesis of Schrock carbenes
Usually from decomposition of alkyls
(a-elimination)
Variations:
Ta
5
Np3Ta
Cp2TaMe3 Cp2TaMe2
+ Cp2Ta
CH2
Me
E+
B-
Cp2Ta
Me
Cp2Ta
Me
Cp2Ta
CH2
H

6. Reactivity of Schrock carbenes
• With ketones: "Wittig"
• With electrophiles: attack on carbene carbon
Unhindered carbenes dimerize:
• With olefins: metallacycles
– Metathesis catalysis !
– "intermediate" carbenes are better for this
M C
O C
+
M
O
C
C
+
M C
C M
+
M
C M
C
M C
C C
+
M
C C
C

7. Fischer-type carbene complexes
• Ligand best described as strong s-donor,
medium-weak p-acceptor
• Carbene has singlet ground state
– because of p-donor substituents
• Metal: late, low oxidation state
– count carbene as neutral ligand
• Carbene carbon is electrophilic
• pi-donor substituents on the carbene atom such as alkoxy and
alkylated amino groups.
• pi electron acceptor metal ligands
• Prototype:
– nucleophilic attack
at coordinated CO
p
s
M C
p
s
M
M
(CO)5W
OMe
Me
W(CO)6
1) MeLi
2) MeI

8. Synthesis of Fischer carbenes
• Usually from CO or RNC complexes via
1) Nucleophilic attack on C
2) Electrophilic attack op O/N
• Also: spontaneous isomerization of vinyl, olefin of acetylene
complexes (Ru)
MeI
MeLi
W(CO)6
(CO)5W
OMe
Me
(CO)5W
O Li
Me

9. Reactivity of Fischer carbenes
• Decomposition:
a) Dimer of carbene (C=C)
b) Olefin, via isomerization of carbene
• Oxidation to ketone
• Nucleophilic substitution on carbene carbon

10. Applications
• heterogeneous catalyses for alkene metathesis in the Shell
higher olefin process.
• A variety of related reactions are used to interconvert light
alkenes, e.g. butenes, propylene, and ethylene.
• Carbene-complexes are invoked as intermediates in the
Fischer-Tropsch route to hydrocarbons.
• A variety of soluble carbene reagents, especially the Grubbs'
and molybdenum-imido catalysts have been applied to
laboratory-scale synthesis of natural products and materials
science.
• In the nucleophilic abstraction reaction, a methyl group can be
abstracted from a Fischer carbene for further reaction.