A novel heteropolytungstate dimer [(SiW10O37)(2)Fe-4(OH)(4)](12-) formed by connecting two alpha-SiW10Fe2O39 units by two hydroxo bridges. Synthesis and crystal structure of its rubidium salt.

The heteropolyanion [(SiW10O37)(2)Fe-4(OH)(4)](12-) is obtained by reaction of iron(III) nitrate on potassium undecatungstosilicate. This reaction lea ds, in a first step, to a ferri-unclecatungstosilicate which dimerizes in n eutral medium; in a second step, substitution of an iron for a tungsten ato m occurs. The compound is isolated as a rubidium salt Rb-12(SiW10O37)(2)Fe- 4(OH)(4). 28 H2O and characterized by elemental analysis, infrared spectros copy and polarography. The novel polyoxoanion can be described as a dimer o f Keggin ions. Two lacunary [alpha-SiW10O37](10-) units, which accommodate two Fe3+ ions each, are linked through two bridging hydroxo groups. The loc ating of the hydroxo groups is done by the bond valence sum calculations. T he tide compound crystallizes in a monoclinic unit cell with lattice consta nts a = 12.613(9), b = 35.858(9), c = 23.317(5) Angstrom, alpha = 93.58(3)d egrees, V = 10 538(8) Angstrom(3) and Z = 4. The structure is solved by dir ect methods. The final reliability factors R and R-w are respectively equal to 0.0775 and 0.0870. (C) 2000 Academie des sciences / Editions scientifiq ues et medicales Elsevier SAS.