The chemical bond in gold(I) complexes with N-heterocyclic Carbenes

In this contribution we report a comparative analysis of the chemical bond between an N-heterocyclic carbene and different Au(I) metal fragments of general formula [(NHC)AuL]⁺ or [(NHC)AuL], where NHC is imidazol-2-ylidene and L is chosen from some ligands frequently used both in coordination and in organometallic chemistry. The focus is on the nature of the Au(I)-C (of NHC) bond in terms of Dewar-Chatt-Duncanson components and its modulation by the ancillary ligand L. In the case of L = Cl (metal fragment AuCl), we present a comparative analysis of the binding mode with 1,3-dimethylimidazol-2-ylidene and 13-diphenylimidazol-2-ylidene, where the hydrogens bonded at the nitrogens of NHC have been substituted with methyl and phenyl groups. We applied a model-free definition and a theoretical analysis of the electron-charge displacements making up the donation and back-donation components of the Dewar-Chatt-Duncanson model. We thus show that the nature of the NHC-gold bond is strongly dependent on the electronic structure of the ancillary ligand L. The results clearly confirm that the NHC is not a purely -donor for our systems, but has a π-back-donation component that amounts to up to half of the -donation (as found in NHC-AuCl) or is entirely negligible (as found in [NHC-AuCO] ⁺).