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 halogenbridged geometry, with the gold atom in an almost square-planar coordination. The geometrical parameters from electron diffraction (r g and ∠ R ) 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 2V -symmetry structures due to Jahn-Teller distortion. CASSCF calculations show that the triplet D 3h -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 4ion has also been calculated, the latter both in planar, D 4h , and tetrahedral, T d , arrangements. The tetrahedral configuration of AuCl 4is subject to Jahn-Teller effect, which leads to a complicated potential energy surface. The factors leading to the planar geometry of AuCl 4and 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.

The molecular geometry of monomeric and dimeric gold trifluoride, AuF 3 and Au 2 F 6 , has been determined by gas-phase electron diffraction and high-level quantum chemical calculations. Both experiment and computation indicate that the ground-state structure of AuF 3 has C 2V symmetry, rather than 3-fold symmetry, with one shorter and two longer Au-F bonds and an almost T-shaped form, due to a first-order Jahn-Teller effect. CASSCF calculations show the triplet D 3h symmetry structure, 3 A′, to lie about 42 kcal/mol above the 1 A 1 symmetry ground state and the D 3h symmetry singlet, 1 A′, even higher than the triplet state, by about a further 13 kcal/mol. The molecule has a typical "Mexican-hat"-type potential energy surface with three equal minimum-energy structures around the brim of the hat, separated by equal-height transition structures, about 3.6 kcal/mol above the minimum energy. The geometry of the transition structure has also been calculated. The dimer has a D 2h symmetry planar, halogen-bridged geometry, with the gold atom having an approximately square-planar coordination, typical for d 8 transition metals. The geometries of AuF and Au 2 F 2 have also been calculated. The very short Au‚‚‚Au separation in Au 2 F 2 is indicative of the so-called aurophilic interaction. This effect is much less pronounced in Au 2 F 6 .

A new high-temperature gas-phase electron diffraction analysis of manganese trifluoride, combined with high-level quantum chemical calculations, provides direct geometrical evidence of a Jahn−Teller distortion in a free molecule. The potential energy surface of the molecule was scanned extensively by computation at the SCF level. CASSCF and CASPT2 calculations established that of the many C 2 v symmetry stationary points on the potential energy surface the lowest energy ones are quintets. The global minimum is a quintet state of 5A1 symmetry. In this planar C 2 v symmetry structure there are two longer and one somewhat shorter Mn−F bonds, with two bond angles close to 106° and one bond angle of about 148°. The second lowest energy state was of 5B2 symmetry, which turned out to be a transition state with an imaginary b2 (in plane) bending frequency. A constrained planar structure of D 3 h molecular symmetry has appreciably higher energy than the 5B2 symmetry structure. The experimental data are in complete agreement with the results of the computations in giving the best agreement with a C 2 v structure characterized by r g (Mn−F) = 1.728 ± 0.014 Å (once), r g(Mn−F) = 1.754 ± 0.008 Å (twice), ∠α(F−Mn−F) = 106.4 ± 0.9° (twice), and ∠α(F−Mn−F) = 143.3 ± 2.0° (once). Our computed infrared wavenumbers and intensities make an assignment possible for published vibrational spectra of MnF3.