N-heterocyclic carbene, silylene, and germylene complexes of MCl (M = Cu, Ag, Au). A theoretical study

Quantum chemical calculations at the MP2 level of theory using relativistic ECPs with large valence basis sets for the metals are reported for the complexes of CuCl, AgCl, and AuCl with the N-heterocyclic carbene imidazol-2-ylidene I and the related silylene 2 and germylene 3. The metal-ligand bond dissociation energies are predicted at CCSD(T). The metal-carbene bonds are very strong. The strongest bond is predicted for 1-AuCl, which has a bond strength D-e = 82.8 kcal/mol. Even the silylene and germylene complexes have substantial bond energies between 37.4 and 64.1 kcal/mol for 2 and between 29.9 and 49.4 kcal/mol for 3. The trend of the bond energies for the metal fragments is AuCl > CuCl > AgCl, and for the ligands it is 1 > 2 > 3. The metal-ligand bonds have a strong ionic character which comes from the Coulomb attraction between the positively charged metal atom and the sigma-electron pair of the donor atom. The covalent part of the bonding shows little pi-back-bonding from the metal to the ligand. The aromaticity of the N-heterocyclic ligands is slightly enhanced in the metal complexes.