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.