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