New Compounds Containing the Complex Anion [Ta2S11]4−; A4Ta2S11 (A = Rb, Cs)

The new tetra(alkali metal) ditantalum undecasulfides A4Ta2S11 (A = Rb, Cs) were prepared at low temperature via the molten-flux method. The crystal structures consist of well separated A+ and [Ta2S11]4− ions. Within the anions, the Ta atoms display a sevenfold coordination by S22− units and S2− anions; the coordination polyhedra can be described as distorted pentagonal bipyramids. Two such bipyramids share common faces, thus forming the [Ta2S11]4− anion.