Molecular Structure, Bonding, and Jahn−Teller Effect in Gold Chlorides: Quantum Chemical Study of AuCl3, Au2Cl6, AuCl4-, AuCl, and Au2Cl2 and Electron Diffraction Study of Au2Cl6

The molecular geometry of dimeric gold trichloride has been determined by gas-phase electron diffraction and high-level quantum chemical calculations. The molecule has a planar, D 2 h -symmetry halogen-bridged geometry, with the gold atom in an almost square-planar coordination. The geometrical parameters from electron diffraction ( r g and ∠ α ) are: Au−Cl t, 2.236 ± 0.013 Å; Au−Cl b, 2.355 ± 0.013 Å; ∠Cl t −Au−Cl t, 92.7 ± 2.5°; and ∠Cl b −Au−Cl b, 86.8 ± 1.8° (t, terminal; b, bridging chlorine). Quantum chemical calculations have also been carried out on the ground-state and transition-state structures of monomeric AuCl 3; both have C 2 v -symmetry structures due to Jahn−Teller distortion. CASSCF calculations show that the triplet D 3 h -symmetry structure lies ∼29 kcal/mol above the 1 A 1 symmetry ground state. The Mexican-hat-type potential energy surface of the monomer has three equal minimum-energy positions around the brim of the hat, separated by three transition-state structures, ∼6 kcal/mol higher in energy, at the CASSCF level. The distortion of AuCl 3 is smaller than that of AuF 3, and the possible reasons are discussed. The structure of the AuCl 4 - ion has also been calculated, the latter both in planar, D 4 h, and tetrahedral, T d, arrangements. The tetrahedral configuration of AuCl 4 - is subject to Jahn−Teller effect, which leads to a complicated potential energy surface. The factors leading to the planar geometry of AuCl 4 - and Au 2 Cl 6 are discussed. The frequently suggested dsp 2 hybridization as a possible cause for planarity is not supported by this study. The geometries of AuCl and Au 2 Cl 2 have also been calculated. The very short Au···Au distance in the latter, similarly to Au 2 F 2, is indicative of the aurophilic interaction.