S tructure of Dibromotetrakis(dimethyl sulfo xide)osmium(II )

OsBr2(C2H6OS)4), M r = 662.52, tetragonal, I4/m, a = 9.178 (2), c= 11.126 (2)A, V= 937.2 (4) A 3, z = 2, D x = 2.35 g cm -3, 2(Mo Ka) = 0.71069A, /~= 114.66 cm -1, F(000)= 628, T= 296 K, R = 0.024 for 425 unique reflections. Crystals were prepared by warming a solution of bis(tetra- n-butylammonium) hexabromoosmate(IV) and dimethyl sulfoxide. The molecule has 4/m symmetry with the Os atom at the 4/m intersection, S and O atoms on the symmetry plane, and the trans-Br atoms on the fourfold axis. All Me2SO ligands are S-bonded to the Os atom. Principal distances and angles are Os-Br = 2.555 (1), Os-S = 2.351 (2), S-O = 1.487 (6), S-C = 1.784 (6)A, Os-S-C = 115.8 (2), Os-S-O = 113.5 (2), o-s-c = 105.2 (3) and C-S-C' = 99.8 (5) °. Experimental. Preparation by dissolution of 50 mg of bis(tetra-n-butylammonium) hexabromoosmate(IV) in 2 ml of dimethyl sulfoxide and heating at 323 K for two weeks. Color of solution changed from wine-red to orange, solution cooled slowly to 253 K, pale-yellow crystals separated by filtration. Crystal used for data collection 0.17 x 0.22 x 0.30 mm, pale-yellow frag- ment cut from larger tetragonal dipyramid. Rigaku AFC-5S diffractometer, graphite-monochromated Mo Ka radiation, o9-20 scans, scan speed 6 ° min -~, maximum of three scan repetitions to obtain aF/F < 0.10. Lattice parameters from least-squares fit of 25 strong reflections in 20 range 49-50 °. 441 reflec- tions measured (h, k 0 to 10, l 0 to 13), 16 reflections considered unobserved with (I< 3o(1)), data set composed of 425 unique reflections, (sin0/2)maz~= 0.60A -~. Three standard reflections (101, 011, 110) changed by -1.2, -1-1 and 0.5%, respectively; no decay correction applied. Data corrected for Lorentz, polarization and absorption (empirical ~, scan correc- tion, three reflections, transmission range 0.43-1.00). Systematic absences indicated space group 14, I2,, or I4/m; structure solved in I2, and later converted to I4/m when the plane of symmetry normal to the fourfold axis became apparent. Direct methods used to locate the Os, Br and S atomic sites, C and O positions from difference Fourier synthesis. Full-matrix least- squares refinement was performed to minimize ~.w( I F o I -- I Fcl )2 where w = ( 1/O"2( I F o I )). Methyl H atomic sites located from difference Fourier synthesis but failed to refine to reasonable positions. H atoms geometrically optimized from difference Fourier peak positions and fixed (C-H = 0.95 A, tetrahedral H-C-H and H-C-S angles), all H atom B's set at 1.2 x Beq of associated C atom. Regeneration of H positions and refinement to convergence repeated until shifts became insignificant. Final refinement performed with 27 variables including all non-H positional and anisotropic thermal parameters, and one scale factor. Convergence yielded R =0.024, wR =0.034, S= 1.14 and (A/tr)max=0"01. Final difference synthesis produced (AP)max = 1.33 and (Ap)min=-0"69eA -3, located near S(1) and Os(1), respectively. Atomic scattering factors and anomalous-dispersion correc- tions from Cromer & Waber (1974). Positional and thermal parameters are listed in Table 1, selected interatomic distances and angles are given in Table 2.t