Steric and electronic effects in the bonding of N-heterocyclic ligands to transition metals

N-heterocyclic (NHC) ligands constitute a new class of ligands that is going to commonly be used in organometallic chemistry. Nevertheless, detailed understanding of the bonding properties of these ligands to transition metals is scarce. In particular, a clear separation between steric and electronic effects is missing. Only in recent years combined experimental and computational studies on this topic have been performed. Here we review some advances in the field. We thus present a quantification of steric effects on the bond dissociation energy of various NHC-ligands from transition metals in complexes as Cp*Ru(NHC)Cl and Ni(CO)₃(NHC). We further compare the steric requirements of various NHC ligands with the steric requirements of some phosphines. In the second part, we examine the different bonding modes that can contribute to the NHC-metal bond. We will review examples of metal-to-NHC back-donation (σ → d*) as well as of ligand-to-metal-to-NHC back-donation (π → d).