| Summary: | The title compound, K11[α-Pt(μ3-OH)2(μ3-OH0.5)Mo6(μ3-O)3(μ2-O)6O12]2·12H2O (simplified chemical formula K11[H2.5PtMo6O24]2·12H2O), containing the well-known Anderson-type heteropolyoxomolybdate anion, was obtained by hydrothermal reaction at pH = ca 6.0. The complete polyanion dimer has 2/m symmetry. The locations of the H atoms with respect to protonated O atoms were obtained from difference Fourier maps, and confirmed by the interpolyanion hydrogen bonds, bond-length elongation and bond-valence sums (BVSs). The title heteropolyanion has two types of protonated O atoms viz. μ3-OH, {Mo2–O(H)–Pt} and μ3-OH0.5 (disordered H atom). The [H2.5α-PtMo6O24]5.5− polyanion forms a dimer, [(H2.5α-PtMo6O24)2]11−, held together by two pairs of μ3-O—H...μ1-O and of one disordered μ3-O...H...μ3-O hydrogen bonds. Three K+ ions are located on special positions (two on twofold rotation axes, one on a mirror plane), and two K+ ions are located on general positions with a reduced occupancy of 0.5. The remaining K+ ion has a reduced occupancy of 0.25 for charge balance and reasonable displacement parameters. As a result, the numbers of K+ and H+ ions in the title compound are 5.5 and 2.5, respectively.
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