Organometallic catalysis has been based for years on phosphine and cyclopentadienyl ligands. These stood alone as the leading ligand classes because they were sterically and electronically tunable to achieve desired levels of catalyst selectivity and activity. In the case of the phosphine series, such tuning was particularly easy using the Tolman ‘map’ that predicted the steric and electronic effects of almost any phosphine or related phosphorus donor ligand (1, 2).

N-Heterocyclic carbenes (NHCs), 1, are ligands formed by the deprotonation of an N,N′-disubstituted imidazolium (or other azolium) salt. Binding of a transition metal to the C2 carbon of the NHC leads to the formation of a very strong metal–carbon bond, the strength deriving from the thermodynamic instability of the free NHC. Unlike metal–carbon bonds in general, those to NHCs do not undergo fast insertion or reductive elimination reactions and so NHCs are relatively reliable spectator ligands. The role of a spectator ligand is to act as a placeholder by promoting a desired reaction at the metal, while avoiding dissociation or entering directly into the reaction. NHCs are significant in being the first new series of spectator ligands in several decades to rise to prominence, having both steric and electronic tunability and the capability to promote catalysis of many useful catalytic reactions.

Early work in the 1960s and 1970s laid out the basis of the field but the full potential of these ligands was not fully realised at that time. Only with work from the 1990s and specially since 2000 have these ligands achieved major prominence. Perhaps the most dramatic example of their utility was the discovery by Grubbs and coworkers (3) that NHC ligands could greatly improve the performance of the Grubbs ruthenium metathesis catalyst.

A recent special issue of Coordination Chemistry Reviews, with guest editor Robert H. Crabtree, has now been devoted to the NHC ligands (4). Among several notable reviews, Ivan Lin and Chandra Vasam address Lin's metallation procedure for the synthesis of NHC complexes. The procedure is important in that it avoids strong base and instead uses Ag₂O to metallate the usual N,N′-disubstituted imidazolium NHC precursor salt. This is now one of the most popular methods of introducing NHCs into metal complexes, because the silver NHC complexes initially formed readily transmetallate to other metals, such as palladium or rhodium.

Polly Arnold and Stephen Pearson describe the chemistry of abnormal NHCs, in which a metal is bound not at the usual C2 carbon but at C4(5). These are much stronger electron donors, but somewhat more easily cleaved from the metal compared with their normal NHC analogues.

Andreas Danopoulos and David Pugh treat ‘pincer’ versions of NHCs, a ligand type that has been very fruitful in terms of catalytic complexes, both in the NHC and in the phosphine series.