Metal carbene

A transition metal carbene complex is an organometallic compound featuring a divalent organic ligand. The divalent organic ligand coordinated to the metal center is called a carbene. Carbene complexes for almost all transition metals have been reported. Many methods for synthesizing them and reactions utilizing them have been reported. The term carbene ligand is a formalism since many are not derived from carbenes and almost none exhibit the reactivity characteristic of carbenes. Described often as M=CR₂, they represent a class of organic ligands intermediate between alkyls (−CR₃) and carbynes (≡CR). They feature in some catalytic reactions, especially alkene metathesis, and are of value in the preparation of some fine chemicals.

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 being 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 valent metals. N-Heterocyclic carbenes (NHCs) were popularlized 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 with regards to its electrophilicity or nucleophilicity.

Fischer carbenes are found with:

The chemical bonding (Scheme 1) is based on σ-type electron donation of the filled lone pair orbital of the carbene atom to an empty metal d-orbital, and π back bonding of a filled metal d-orbital to the empty p-orbital on carbon. An example is the complex (CO)₅Cr=C(NR₂)Ph.

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