Solid State Synthesis and Structural Characterization of the Compound H_2 [(n-Bu)_4N]_3 [As (SiW_(11)O_(39))]

H2 [(n-Bu)4N]3 [As (SiW11O39)], Mr = 3478. 89, cubic, space groupI43m, a=b=c= 17. 785 (8) A, V=5625. 0 A3, Z= 1, Dc= 2. 05 g/cm3, μ(MoKα)= 118. 3 cm-1, F(000) = 3808, final R= 0. 072 and Rw= 0. 083 for 248 observed re-flections with I≥3. 8σ(I). X-ray analysis shows that the heteropolyanion [As (SiW11-O39)]5- contains a central SiO4 unit encapsulated by a cage composed of arsenic, tung-sten and oxygen atoms.