Hydrogen potassium vanadoarsenate ([H3KV12As3O39(AsO4)]6-) and related topological and/or structural aspects of polyoxometalate chemistry

On reduction of an aqueous solution of vanadate(V) with thiocyanide in the presence of As(V), the black-gray mixed-valence compound K6[H3KV12As3O39(AsO4)].8H2O (1) is formed. It crystallizes in the space group C2/c with a = 39.993 (15) angstrom, b = 13.467 (4) angstrom, c = 18.209 (6) angstrom, beta = 111.52 (3)-degrees, and Z = 8. Refinement of 686 variables with 8827 observed reflections yields R = 0.049 and R(w) = 0.058. The compound contains an anion with approximate C3 symmetry, which can be described as a fragment of a sphere capped with a potassium ion. The oxygen and/or metal atoms of polyoxometalates often span the vertices of topologically and/or geometrically interesting polyhedra. Related aspects of 1 are discussed.