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 sigma-donor for our systems, but has a pi-back-donation component that amounts to up to half of the sigma-donation (as found in NHC AuCl) or is entirely negligible (as found in [NHC AuCO])(+).