Synthesis and X-ray study of the polyanion [AsIII4Mo6W14O72(H2O)2]12- isomorphous of [AsIII4W20O72(H2O)2]12-.

The addition of an excess of MoO42- to a solution of [(As2W8O30)-W-III(OH)](7-) led at pH = 4 to a new polyanion [(As4Mo0W14O72)-Mo-III(H2O)(2)](12-) isostructural of [(As4W20O72)-W-III(H2O)(2)](12-). The structure of this compound was determined by X-ray diffraction: a = 11.183(4) Angstrom, b = 13.432(3) Angstrom, c = 19.626(5) Angstrom, alpha = 77.87(2), beta = 73.57(3), gamma = 67.44(3)degrees, V = 2 594(1) Angstrom(3), P (1) over bar. Reliability factors are R = 7.89 and R-w = 9.97. The starting compound underwent an isomerisation beta-->alpha and one tungsten was formally substituted by one molybdenum atom; two other molybdenum atoms were incorporated. The polyanion is formed of two units alpha B-(AsMo2W7O33)-Mo-III connected by a double chain (AsOMo2O2)-O-III(H2O). Arsenic and molybdenum are both bound to two octahedrons WO6 of one unit (AsOMoO2W7O33)-O-III, both molybdenum atoms of which are contiguous. (C) 2000 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.