B3LYP, M06 and B3PW91 DFT assignment of nd8 metal-bis-(N-heterocyclic carbene) complexes.

Abstract

This paper is focused on the examination of the bonding properties of a series of [M(NHC)2X2] (M = nd8 transition metal; X = Cl, Br and I) complexes in normal, abnormal and mixed C∩C coordination modes. Structures have been optimised in gas phase using B3LYP, M06 and P3BW91 functionals. Two basis sets have been used: the LanL2DZ and a mixed basis set (LanL2DZ for nd8 transition metals as well as halogen atoms and 6-311+G(d,p) for other atoms). Results obtained indicate that the B3PW91 bond distances are closer to experimental data. The complexation energies obtained for each binding mode increase in the order: Ni2+ < Pd2+ < Pt2+, independently of the halogen atom adopted. From the Quantum Theory of Atoms in Molecule (QTAIM) approach, the instability has been found to follow this trend: M - X < M - C. The analysis of metal-ligand interactions using the natural bond orbital (NBO) revealed that the strongest metal-ligand interactions are observed in the normal binding mode. The NCH → MX2 donation terms were found to be interestingly predominant compared with back donation ones in the complexes studied, except in Pt chloride ones. The contribution of electrostatic interaction energy between the above fragments (∆Eelstat term) is in the range 57.48-63.95% traducing the fact that the interactions are mostly electrostatic. Graphical abstract.