The Chemical Bond in Gold(I) Complexes with N‑Heterocyclic\nCarbenes

Abstract

In this contribution we report a\ncomparative analysis of the chemical\nbond between an N-heterocyclic carbene and different Au­(I) metal fragments\nof general formula [(NHC)­AuL]⁺ or [(NHC)­AuL], where NHC\nis imidazol-2-ylidene and L is chosen from some ligands frequently\nused both in coordination and in organometallic chemistry. The focus\nis on the nature of the Au­(I)–C (of NHC) bond in terms of Dewar–Chatt–Duncanson\ncomponents and its modulation by the ancillary ligand L. In the case\nof L = Cl (metal fragment AuCl), we present a comparative analysis\nof the binding mode with 1,3-dimethylimidazol-2-ylidene and 13-diphenylimidazol-2-ylidene,\nwhere the hydrogens bonded at the nitrogens of NHC have been substituted\nwith methyl and phenyl groups. We applied a model-free definition\nand a theoretical analysis of the electron-charge displacements making\nup the donation and back-donation components of the Dewar–Chatt–Duncanson\nmodel. We thus show that the nature of the NHC–gold bond is\nstrongly dependent on the electronic structure of the ancillary ligand\nL. The results clearly confirm that the NHC is not a purely σ-donor\nfor our systems, but has a π-back-donation component that amounts\nto up to half of the σ-donation (as found in NHC–AuCl)\nor is entirely negligible (as found in [NHC–AuCO]⁺).