The distorted structure of [Au(GeCl₃)₃]^2-

It is well known that [Au(PR₃)₃]⁺ compounds (R any organic ligand) adopt a trigonal planar AuP₃ arrangement, small distortions being only due to steric repulsion between the ligands R. This is supported by relativistic MP2 geometry optimizations for the free gas phase species which yield the ideal trigonal planar AuP3 structure for the model compound [Au(PH₃)₃]⁺. Model calculations on the recently synthesized compound [Au(GeCl₃)₃]^2- which is isoelectronic to [Au(PR₃)₃]⁺ also reveal a trigonal planar AuGe₃ structure. However, the recently determined X-ray structure of [Au₂(dppm)₂][Au(GeCl₃)₃] shows a T-shaped AuGe₃ arrangement. We demonstrate that this distortion is caused by solid state effects, that is the influence of the counter cations are necessary in order to obtain the observed symmetry breaking. However, unlike AuF₃ which has recently been determined by electron diffraction to be T-shaped in the gas phase caused by a first-order Jahn-Teller effect, this distortion cannot be so easily rationalized by a similar AuGe₃ Jahn-Teller effect along the e′ distortion mode. Model calculations on Na₂[Au(GeCl₃)₃] show that the strong Coulomb interaction between the negatively charged chlorine atoms and the Na⁺ ions leads to a distortion from a trigonal planar to the T-shaped AuGe₃ arrangement lowering the energy by 137 kJ mol^-1.