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-2h-symmetry halogen-bridged geometry, with the gold atom in an almost square-planar coordination. The geometrical parameters from electron diffraction (r(g) and angle (alpha)) are: Au-Cl-t, 2.236 +/- 0.013 Angstrom; Au-Cl-b, 2.355 +/- 0.013 Angstrom; angle Cl-t-Au-Cl-t, 92.7 +/- 2.5 degrees; and angle Cl-b-Au-Cl-b, 86.8 +/- 1.8 degrees (t, terminal; b, bridging chlorine). Quantum chemical calculations have also been carried out on the ground-state and transition-state structures of monomeric AuCl3; both have C-2v-symmetry structures due to Jahn-Teller distortion. CASSCF calculations show that the triplet D-3h-symmetry structure lies similar to 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, similar to6 kcal/mol higher in energy, at the CASSCF level. The distortion of AuCl3 is smaller than that of AuF3, and the possible reasons are discussed. The structure of the AuCl4- ion has also been calculated, the latter both in planar, D-4h, and tetrahedral, T-d, arrangements. The tetrahedral configuration of AuCl4- is subject to Jahn-Teller effect, which leads to a complicated potential energy surface. The factors leading to the planar geometry of AuCl4- and Au2Cl6 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 Au2Cl2 have also been calculated. The very short Au . . . Au distance in the latter, similarly to Au2F2, is indicative of the aurophilic interaction.