Theoretical investigation of the bonding properties of N-heterocyclic carbenes coordinated to electron-rich d8 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.

The π bonding interactions of N-heterocyclic carbenes and pyridine to nickel(II), palladium(II), and platinum(II) centers has been investigated by energy decomposition analysis, atomic charge variation and bond order analysis. The carbene ligand appears to feature significant π backbonding, in particular when bound to electron-rich metals (X = H, in pictogram).Figure optionsDownload as PowerPoint slide