Ligand effects due to resonance character in LAuCCH(-) (L = F, Cl, Br, I, CCH) complexes: an NBO/NRT analysis.

The organogold complexes of LAuCCH(-) (L = F, Cl, Br, I, CCH) were investigated using natural bond orbital/natural resonance theory (NBO/NRT) methods. The NBO/NRT results strongly support the general resonance-type three-center-four-electron (3c/4e) picture of LAuCCH: L(-): Au-CCH ↔ L-Au :CCH(-), arising from hyperconjugation interactions. The sums of ionic and covalent contributions to both L-Au and Au-CCH bonds are all slightly larger than that due to the additional π-back bonding within the 3c/4e hyperbonded triad. This complementary relationship between L-Au and Au-CCH bond orders implies a competing relationship between the ancillary ligand and CCH around the gold atom. We discuss the ligand effects in the LAuCCH(-) series on the basis of this competing relationship.