Static and dynamic structure of Au2Cl6, AuAlCl6 and AuFeCl6 clusters

Fourfold coordination of trivalent metal ions by chlorines in molecular dim ers changes from tetrahedral in Al2Cl6 and Fe2Cl6 to planar in Au2Cl6. With in an ionic model we interpret this transition in the stable molecular shap e as due to the quadrupolar polarizability of the gold(III) ion. We also de monstrate within the same model that a mixed tetrahedral-planar coordinatio n is stable in AuAlCl6 and AuFeCl6 clusters. Quantitative comparisons are p resented for the calculated molecular structures and vibrational frequencie s against the available experimental data from crystalline diffraction and from Raman spectra of the crystal and of vapours.