The reduction of triosmium dodecacarbonyl by sodium borohydride. The preparation and X-ray structure of the cluster anion [H2Os4(CO)12]2−

The reduction of Os3(CO)12 by NaBH4 in tetrahydrofuran has been studied, and the formation of the anionic clusters [HOs3(CO)11]−, [H3Os4(CO)12]− and [H2Os4(CO)12]2− observed. The previously unreported dianion [H2Os4(CO)12]2− was prepared in satisfactory yield, and characterised as the bis(triphenylphosphine)iminium salt. This compound crystallizes in the space group P 1 , with Z = 1, and cell dimensions of a 11.014(2), b 14.751(3), c 15.168(3) Å, α 123.95(2)°, β 95.77(2)°, γ 98.73(2)°. The structure was solved by a combination of multisolution sign expansion and Fourier methods, and final residuals were R 0.067 and R W 0.066 for 5972 observed intensity data. The dianion comprises a distorted tetrahedron of osmium atoms, each metal also bonding to three terminal carbonyl ligands, which as staggered with respect to the metalmetal bonds. Unlike the cation, the cluster anion is statistically disordered between two centrosymmetrically related sites.