Theoretical investigation of the bonding properties of N-heterocyclic carbenes coordinated to electron-rich d metal centers

Density functional theory (DFT) at the generalized gradient approximation (GGA) level has been applied for the analysis of the bond between group 10 metals and N-heterocyclic carbene (NHC) in complexes MX₃(NHC)]⁻ (M = Ni, Pd, Pt, X = H, Cl, I). For comparative purposes, similar calculations have been performed for analogous pyridine complexes MX₃(py)]⁻ (py = pyridine). Full geometry optimizations have been performed for all complexes. The role of the M-L π interaction was investigated by the aid of respectively, energy decomposition analysis, Hirshfeld atomic charge variation, molecular orbital considerations and bond order decomposition analysis. The π-bonding contribution increases in the order I < Cl < H, and Pt < Pd < Ni. Most significantly, the absolute π-acceptor ability of the NHC in these complexes is larger than that of pyridine. However, due to the dominant σ donor interactions, the relative contribution, that is the π/σ ratio, is predicted to be smaller.