Donor-acceptor properties of ligands from the natural orbitals for chemical valence

Natural orbitals for chemical valence (NOCV) have been used to characterize donor-acceptor properties of ligands in model nickel(II) complexes. NOCV allows for separation of ligand -> metal and metal -> ligand electron transfer processes (Dewar-Chatt-Duncanson model). Bonding between the ligand X = CN-, PH3, NH3, C2H4, CO, CS, N-2, NO+ and the metal-containing fragment in the [Ni L-3](2+) complexes (L = NH3, CO) have been discussed. For both sigma-donation and pi-back-bonding, the resulting orders of ligands are in a qualitative agreement with those commonly accepted. However, it was also demonstrated that the influence of the metal-containing fragment can be substantial, changing the relative donor-acceptor characteristics of different ligands.