Stability and reactivity of N-heterocyclic carbene complexes

The use of N-heterocyclic carbenes as ligands for transition metals has increased dramatically in the last few years, spurred on by their remarkable successes in the areas of metathesis chemistry and coupling reactions. The stability of the transition metal complexes of N-heterocyclic carbenes is often cited as one of the key advances of these ligands versus their phosphine counterparts. However, to quote Danopoulos, recent reports are questioning the belief that the NHC-metal bond is inert [Chem. Commun. (2003) 756]. This review describes some of the reactions that N-heterocyclic carbenes undergo in the metal co-ordination sphere that lead to decomposition of the NHC-metal complex. lmplications for catalysis using these species are described. (C) 2004 Elsevier B.V. All rights reserved.