The Crystal Structure of Guanidinium Hexamolybdobis(phenylarsonate) Tetrahydrate, (CN3H6)4[(C6H5As)2Mo6O25H2]·4H2O

Two heteropoly anions, [C6H5AsMo6O21(H2O)6]2− and [(C6H5As)2Mo6O25H2]4−, have been synthesized by the reaction of phenylarsonic acid with molybden um trioxide in an aqueous solution of pH ca. 2. The latter anion has been subjected to crystal-structure analysis as a guanidinium salt, (CN3H6)4[(C6H5As)2Mo6O25H2]·4H2O, and a heteropoly anion with a new type of structure has been established. The crystal is monoclinic with the space group of P21 and cell dimensions of a=13.754(5), b=15.752(7), c=11.900(5) Å, β=115.59(2)°, V=2325.3 Å3, Dm=2.25, Dx=2.28 g cm−3, and Z=2. The formula of the anion has been proved to be [(C6H5As)2Mo6O25H2]4−, which is different from the expected formula, [(C6H5As)2Mo6O24]4−, though the latter had previously been proposed on the basis of a 17O NMR study and an independent X-ray investigation of the methyl derivative had confirmed the existence of the [(CH3As)2Mo6O24]4− anion. The present anion, [(C6H5As)2Mo6O25H2]4−, consists of six MoO6 octahedra, which form a ring with one face-, two corner- and three edge-sharings, the two C6H5AsO3 tetrahedra are attached to the ring from above and below, with their three oxygens shared with molybdenums. The two phenyl groups are oriented upwards and downwards from the ring respectively.