A transition metal carbene complex is an organometallic compound featuring a divalent carbon ligand, itself also called a carbene.[1] Carbene complexes have been synthesized from most transition metals and f-block metals,[2] using many different synthetic routes such as nucleophilic addition and alpha-hydrogen abstraction.[1] The term carbene ligand is a formalism since many are not directly derived from carbenes and most are much less reactive than lone carbenes.[2] Described often as =CR2, carbene ligands are intermediate between alkyls (−CR3) and carbynes (≡CR). Many different carbene-based reagents such as Tebbe's reagent are used in synthesis. 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.[1][3][4]