Preparation and crystal structures of the new gold rich thioaurates Rb4Au6S5 and Cs4Au6S5

The new thioaurates Rb4Au6S5 and Cs4Au6S5 were prepared by reacting the binary alkali sulphides (Rb2S and Cs2S respectively) with stoichiometric amounts of gold and sulphur at 500 degrees C. Rb4Au6S5 is hexagonal, space group P (6) over bar 2c with a = 9.983(6) Angstrom, c = 9.950(7) Angstrom, Z = 2; Cs4Au6S5 is trigonal, space group P31m, with a = 10.243(3) Angstrom, c = 5.290(2)Angstrom, Z = 1. The crystal structures were determined from single crystal data (Mo K alpha) and refined to conventional R factors of 0.023 (203 F(o)s, 28 variables) for Rb4Au6S5 and 0.048 (453 F(o)s, 28 variables) for Cs4Au6S5. Though both thioaurates are characterized by essentially linear chalcogen coordinations of the gold atoms, the topology of the complex anions formed is entirely different. Rb4Au6S5 contains discrete anionic clusters [Au6S5](4-) of trigonal bipyramidal shape with mu(3)-S on the apices and mu(2)-S forming in the equatorial plane ((d) over bar(Au-S) = 2.32 Angstrom, angle S-Au-S = 170.6 degrees). The gold atoms in the cluster form a trigonal prism with shortest Au-Au contacts of 3.163 Angstrom. The caesium compound, however, is characterized by corrugated layers (2)(infinity)-Au6S54-, consisting of condensed Au12S12 rings with mu(3)-S at the nodal positions ((d) over bar(Au-S) = 2.3 Angstrom, angle S-Au-S = 177.8 degrees). Each gold atom has one close homonuclear neighbour at a distance of only 3.013 Angstrom. Irrespective of their fundamental topological differences, both structures can be coherently described with a geometrical model based on a mixed alkali-sulphur packing.