N-heterocyclic carbenes of the late transition metals: a computational and structural database study

A combined computational chemistry/crystallographic database analysis of the bonding in late transition metal N-heterocyclic carbene complexes (NHCs) is reported. The metal–carbon bond in these complexes is approximately 4% shorter than a prototypical M–C single bond, e.g., as in a metal–alkyl complex. Two hypotheses are investigated for this bond shortening – multiple-bond character in the metal–carbon linkage of the NHC complex, and a change in the hybridization of the carbenoid carbon to incorporate more p character. The results of this research support the latter hypothesis. The natural bond order analysis also suggests a substantial trans influence for NHC ligands.