Coordination’s preference and electronic structure of N -heterocyclic carbene–monometallic complexes: DFT evaluation of σ -bonding and π -backbonding interactions

The geometrical parameters, the electronic structures and the nature of the chemical bonding for complexes of the type (NqMes)M[Cl(CO)2] (NqMes = naphtoquinone-annulated NHC ligand and M = V, Mn, Re, Co, Rh, Cu) which possess even electron counts have been investigated at the BP86 and B3LYP levels of theory using orbital molecular and energy decomposition analyses. The coordination of the six-membered ring is strongly disfavoured (structures A), while the structures B1 and B2 which differ by the orientation of the M[Cl(CO)2] unit are in competition with regard to the metal’s nature and the spin state. The carbene–metal bonds are governed chiefly by electrostatic attractions contributing 65–75% of the bonding attractive interactions. The carbene–metal bonds are strong in electron-moderately rich Re element and weak in electron-rich Cu metal. In the most cases, orbital interactions are more important in symmetrical species than in the unsymmetrical ones, which are mainly described by σ-bonding with the π-backbonding part less than 20%.