How Important Is Backbonding in Metal Complexes Containing N-Heterocyclic Carbenes? Structural and NBO Analysis

DFT calculations have been used to analyze the degree of metal-to-ligand backbonding in a range of model transition metal complexes of N-heterocyclic carbenes. Two new methods of analysis have been introduced for this purpose. The first is structural and involves comparing the variation of the C-N bond length and N-C-N angle in complexes in which s-donation is dominant (e.g. complexes with BF3) and others in which backbonding might be expected. The second uses the perturbative natural bonding orbital (NBO) method. Both methods lead to the firm conclusion that backbonding is significant in these compounds - as large or even larger than in the corresponding phosphane complexes. This is partly due to the very strong sigma-donor nature of the ligand, which makes typical metal acceptor centers very electron-rich and hence very apt to undergo backbonding.