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, N2, NO+ and the metal-containing fragment in the [Ni L3]2+ complexes (L = NH3, CO) have been discussed. For both σ-donation and π-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.