Preparation and Crystal Structures of the New Gold-Rich Thioaurates Rb4Au6S5 and Cs4Au6S5.

Abstract 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°C. Rb4Au6S5 is hexagonal, space group P 6 2c with a = 9.983(6) A , c = 9.950(7) A , Z = 2 ; Cs4Au6S5 is trigonal, space group P31m, with a = 10.243(3) A , c = 5.290(2) A , Z = 1 . The crystal structures were determined from single crystal data (Mo Kα) 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 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 μ3-S on the apices and μ2-S forming in the equatorial plane ( d ( AuS ) = 2.32 A , ∠SAuS = 170.6°). The gold atoms in the cluster form a trigonal prism with shortest AuAu contacts of 3.163 A. The caesium compound, however, is characterized by corrugated layers 2x-Au6S54−, consisting of condensed Au12S12 rings with μ3-S at the nodal positions ( d ( AuS ) = 2.3 A , ∠SAuS = 177.8°). Each gold atom has one close homonuclear neighbour at a distance of only 3.013 A. Irrespective of their fundamental topological differences, both structures can be coherently described with a geometrical model based on a mixed alkali-sulphur packing.