Compounds having multiple bonds between elements are important for understanding the fundamentals of chemistry and also noteworthy from the point of view of substance conversion, as these multiple bond moieties are highly reactive.

The term “carbene” is applied to organic compounds described as CR₂. A transition metal complex having this as a ligand is referred to as a “carbene complex”, and may be written MCR₂ with a double bond between M and C. A transition metal complex with a triple bond between M and C (MCR) is referred to as a “carbyne complex”.

The first carbene complex was prepared by Chugaev in 1925. He reported the preparation of the compound shown in Figure 5.1(a),¹  but 45 years later it was found that this compound was actually the carbene complex shown in Figure 5.1(b).²