Crystal and molecular structure of bis[µ-oxo-oxo(tetramethylethane-1,2-diolato)osmium(VI)]; a dimeric monoester complex with squarepyramidal co-ordination

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Abstract
The crystal structure of the title complex has been determined from X-ray diffractometer data, and has been refined to R 0.038 for 1 309 independent reflections. Crystals are monoclinic, with unit-cell dimensions a= 8.145(3), b= 14.008(3), c= 7.811(3)Å, β= 93.24(3)°; space group P21/n, Z= 2. This structure differs from those hitherto accepted for osmium(VI) monoester complexes in that it is dimeric and has (S2) crystallographic symmetry. A square-pyramidal co-ordination is found for osmium, two of the basal oxygen atoms being from a chelate ester [mean Os–O(ester) 1.87 Å], and the other two from a dioxo-bridge [mean Os–O(br) 1.92 Å]. A terminal oxo-ligand occupies the apical position, with a short Os–O(term) distance of 1.675 Å. Electronic repulsion by this π-donating ligand causes the large O(apical)–Os–O(basal) mean angle of 111.1°, such that the osmium atom is 0.68 Å above the basal plane. The monoester is discussed as a model compound for the staining and fixation of unsaturated membrane lipids by osmium tetroxide.

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