Heteropolyanions of Methylarsonate and Dimethylarsinate: The Crystal Structures of Guanidinium Hexamolybdomethylarsonate Hexahydrate, (CN3H6)2[CH3AsMo6O21(H2O)6]·6H2O and Guanidinium Tetramolybdodimethylarsinate Monohydrate, (CN3H6)2[(CH3)2AsMo4O14(OH)]·H2O

The crystal and molecular structures of polymolybdo complexes of methylarsonate and dimethylarsinate have been determined by the X-ray diffraction technique. The crystal of (CN3H6)2[CH3AsMo6O21(H2O)6]·6H2O is trigonal, with the space group R3 and the cell dimensions of a=9.653(6) Å, α=87.01(15)°, and Z=l. The anion is an assemblage of six MoO6 octahedra and a tetrahedral CH3AsO3 group. The six MoO6 octahedra constitute a ring with alternate edge- and corner-sharing, and the tetrahedral CH3AsO3 group joins the anion, with its three oxygen atoms shared with the ring. Each molybdenum atom has two terminal oxygen atoms in the cis position and is coordinated by one water molecule. (CN3H6)2[(CH3)2AsMo4O14(OH)]·H2O crystallizes in the space group P21/c of the monoclinic system, with the cell dimensions of a=8.530(5), b=8.532(4), c=30.121(15) Å, β=95.50(16)°, and Z=4. The Mo4O14(OH) moiety in the [(CH3)2AsMo4O14(OH)]2− anion is formed with two face- and two edge-sharings of MoO6 octahedra into an almost flat rectangle. The oxygen atom located at the center of the anion and bonded to all the four molybdenum atoms is protonated and is, in fact, a hydroxyl group.