How important is metal-ligand back-bonding toward YX3 ligands (Y = N, P, C, Si)? an NBO analysis

An NBO second-order perturbative energy analysis is used in a quantitative study of back-bonding toward YX3 (Y = P, N, C, Si; X = H, F, Me, Ph, OMe) ligands as well as pyridine and bipyridine. The pi acceptor character of these ligands in M-L, L'-M-L, and M(CO)(5)L complexes is studied at the B3LYP level of theory. All phosphine ligands are found to be pi acceptors, whereas the NH3 and NMe3 ligands are found to be sigma-only ligands. The NBO analysis also identifies competitive back-bonding and shows that even the carbon- and silicon-containing ligands have some pi-bonding character.