NHC ligands versus cyclopentadienyls and phosphines as spectator ligands in organometallic catalysis

N-heterocyclic carbenes are compared with cyclopentadienyls and phosphines in terms of bonding and reactivity. Synthetic difficulties that currently prevent the general incorporation of NHCs into chelate, pincer and tripod ligand architectures are related to the inability of the NHC to reversibly decoordinate to correct binding ‘errors’ in the initial kinetic products of NHC complex formation. The strengths and weaknesses of current synthetic approaches, such as Lin’s Ag2O transmetallation route, are discussed. Linker dependent reactivity patterns are related to azole ring orientation effects and some aspects of cyclometalation are considered.