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=CR
2, they represent a class of organic ligands intermediate between alkyls (-CR
3) and carbynes (=CR). They feature in some catalytic reactions, especially
alkene metathesis, and are of value in the preparation of some fine chemicals.