Transition metal carbene complex

Metal carbene complexes are often classified into two types. The Fischer carbenes, named after Ernst Otto Fischer, feature strong π-acceptors at the metal and are electrophilic at the carbene carbon atom. Schrock carbenes, named after Richard R. Schrock, are characterized by more nucleophilic carbene carbon centers; these species typically feature higher oxidation state (valency) metals. N-Heterocyclic carbenes (NHCs) were popularized following Arduengo's isolation of a stable free carbene in 1991. Reflecting the growth of the area, carbene complexes are now known with a broad range of different reactivities and diverse substituents. Often it is not possible to classify a carbene complex solely with regards to its electrophilicity or nucleophilicity. Schrock carbenes . The distances are 2.37 and 2.04 Å, respectively. Color code: blue = Ta, gray = C, white = H. Schrock carbenes do not have π-accepting ligands on the metal centre. They are often called alkylidene complexes. Typically this subset of carbene complexes are found with: Bonding in such complexes can be viewed as the coupling of a triplet state metal and triplet carbene, forming a true double bond. Both the metal and carbon atom donate 2 electrons, one to each bond. 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. These bonds are weakly polarized towards carbon and therefore the carbene atom is a nucleophile. Furthermore, the major resonance structures of Schrock carbene put the negative charge on the carbon atom, making it nucleophilic. They are popular because they are more readily prepared than Schrock and Fischer carbenes. In fact, many NHCs are isolated as the free ligand, since they are persistent carbenes. Being strongly stabilized by π-donating substituents, NHCs are powerful σ-donors but π-bonding with the metal is weak. For this reason, the bond between the carbon and the metal center is often represented by a single dative bond, whereas Fischer and Schrock carbenes are usually depicted with double bonds to metal. Continuing with this analogy, NHCs are often compared with trialkylphosphine ligands. 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. Bimetallic carbene complexes An early example of this bonding mode was provided by prepared from diazomethane: : Another example of this family of compounds is Tebbe's reagent. It features a methylene bridge joining titanium and aluminum. == Application of Metal Carbenes ==

Metal carbene complexes have applications in hetereogeneous and homogeneous catalysis, and as reagents for organic reactions. Catalysis The dominant application of metal carbenes involves none of the above classes of compounds, but rather heterogeneous catalysts used for alkene metathesis for the synthesis of higher alkenes. 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. Wulff-Dötz Reaction Fischer carbenes are used with alkynes as the starting reagents for the Wulff–Dötz reaction, forming phenols. == History ==

, was not recognized as such until decades after its preparation. The first metal carbene complex to have been reported was Chugaev's red salt, first synthesized as early as 1925, although it was never identified to be a carbene complex. In 1968, Hans-Werner Wanzlick and Karl Öfele separately reported metal-bonded N-heterocyclic carbenes. The synthesis and characterization of ((CH3)3CCH2)Ta=CHC(CH3)3 by Richard R. Schrock in 1974 marked the first metal alkylidene complex. 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. == See also ==