Transition metal carbene complex

They also feature in catalytic reactions, especially alkene metathesis, and are of value in both industrial heterogeneous and in homogeneous catalysis for laboratory- and industrial-scale preparation of fine chemicals.

[5] Reflecting the growth of the area, carbene complexes are now known with a broad range of different reactivities and diverse substituents.

The major resonance structures of Fisher carbenes put the negative charge on the metal centre, and the positive on the carbon atom, making it electrophilic.

Since there is no donation to the carbene atom from adjacent groups, the extent of pi backbonding is much greater, giving a strong double bond.

Furthermore, the major resonance structures of Schrock carbene put the negative charge on the carbon atom, making it nucleophilic.

[12][13] Being strongly stabilized by π-donating substituents, NHCs are powerful σ-donors but π-bonding with the metal is weak.

Like phosphines, NHCs serve as spectator ligands that influence catalysis through a combination of electronic and steric effects, but they do not directly bind substrates.

[17] Metal carbene complexes have applications in hetereogeneous and homogeneous catalysis, and as reagents for organic reactions.

[4] Homogeneous Schrock-type carbene complexes such as Tebbe's reagent can be used for the olefination of carbonyls, replacing the oxygen atom with a methylidene group.

[6] Diazo compounds like methyl phenyldiazoacetate can be used for cyclopropanation or to insert into C-H bonds of organic substrates.

[9] In 1991, Anthony J. Arduengo synthesized and crystallized the first persistent carbene, an NHC with large adamantane alkyl groups, accelerating the field of N-heterocarbene ligands to its current use.

Example Fisher carbene with chromium(0) centre and diethylamine donor.
Orbital interaction in a Fisher carbene. The carbene electrons are donated to a sigma bond, and weak pi-backbonding occurs.
Major resonance structures of (CO) 5 W=COMePh . Structures with a positive charge on carbon are significant and make the carbon electrophilic.
Structure of (C 5 H 5 ) 2 TaCH 3 (CH 2 ) , as determined by X-ray crystallography . [ 8 ] The Ta−CH 3 and Ta=CH 2 distances are 2.37 and 2.04 Å, respectively. Color code: blue = Ta, gray = C, white = H.
Orbital interaction in the bonding of a Schrock carbene. Both the metal and carbon provide 2 unpaired electron each, forming the double bond.
Major resonance structures of a Schrock carbene. The negative charge at the carbon atom renders it nucleophilic.
IMes is a common NHC ligand.
Catalytic cycle of olefin metathesis. The metal complex alternated between a metallocyclobutane ring and carbene complex, catalyzing the formation of new carbon-carbon double bonds.
Olefination of an ester using Tebbe's reagent as a methylidene source.
Nucleophilic abstraction of the methyl group of a Fisher carbene. The negatively charge oxygen is a nucleophile which can undergo further reaction.
Catalytic cycle for the insertion of carbenes into carbon-hydrogen bonds. The metal carbene is generated by nitrogen elimination from the diazo compound, and then inserts into the C-H bond.
General reaction scheme for the Wullf-Dötz reaction, making phenols from Fisher carbene complexes and alkynes.
The first metal carbene complex, Chugaev's red salt , was not recognized as such until decades after its preparation. [ 21 ]