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

The new thioaurates Rb4Au6S5 and Cs4AuS5 were prepared by reacting the binary alkali sulphides (Rb2S and Cs2S respectively) with stoichiometric amounts of gold and sulphur at 500?C. Rb4Au6S5 is hexagonal, space group P`62c with a=9.983(6)Ä, c=9.950(7)Ä, Z=2; Cs4Au6S5 is trigonal, space gorup P31m, with a=10.243(3)Ä, c=5.290(2)Ä, Z=1. The crystal structures were determined from single crystal data (MoK alpha) and refined to conventional R factors of 0.023 (203 Fos, 28 variables) for Rb4Au6S5 and 0.048 (453 Fos, 28 variables) for Cs4Au6S5. Though both thioaurates are characterized by essentially linear chalcogen coordinates of the gold atoms, the topology of the complex anions formed is entirely different. Rb4Au6S5 contains discrete anionic clusters [Au6S5]4- of trigonal bipyramidal hape with ?3-S on the apices and ?2-S forming in the equatorial plane [`d(Au-S)=2.32Ä, < S-Au-S=170.6?). The gold atoms in the cluster form a trigonal prism with shortest Au-Au contacts of 3.163Ä. The caesium compound, however, is characterized by corrugated layers 2-oo-(Au6S5)4-, consisting of condensed Au12S12 rings with ?3-S at the nodal positions (`d(Au-S)=2.3Ä, < S-Au-S=177.8?). Each gold atom has one close homonuclear neighbour at a distance of only 3.013Ä. Irrespective of their fundamental topological differences, both structures can be coherently described with a geometrical model, based on a mixed alkali-sulphur packing