TERT-BUTYLIMIDO COMPLEXES OF OSMIUM-(VIII), OSMIUM-(VII) AND OSMIUM-(VI) - X-RAY CRYSTAL-STRUCTURES OF [(BUTN)2OS(MU-NBUT)2OS(NBUT)(MU-O)]2, [OS(NBUT)2(MU-NBUT)]2 AND [OS(NBUT)2(MU-NBUT)]2[BF4]2

The interaction of OsO4 with neat NHBu(t)(SiMe3) gives the first homoleptic osmium(VIII) imido compound, Os(NBu(t))4 1, together with the tetranuclear osmium(VI) oxo-imido complex [(Bu(t)N)2-Os(mu-NBu(t))2Os(NBu(t))2Os(NBu(t)(mu-O)]2 2. Complex 1 can be reduced to the osmium(VI) dimer, [Os(NBu(t))2(mu-NBu(t))]2 3, by triphenylphosphine or by sodium amalgam in tetrahydrofuran; with trimethyloxonium tetrafluoroborate an unusual reduction reaction leads to the osmium(VII) dimeric cationic complex [Os(NBu(t))2(mu-NBu(t))]2[BF4]2 4. The syntheses of Os(NR)3, R = 2,6-Me2C6H3 5, or 2,6-Pr(i)2C6H3 6, by interaction of OsO4 with NHR(SiMe3) are reported. The X-ray crystal structures of 2, 3 and 4 have been determined. Compound 2 has a chain tetrameric structure with two (Bu(t)N)2Os(mu-NBu(t))2Os(NBu(t))(mu-O) units linked by a dioxo bridge. The outer Os atoms are four-co-ordinate, distorted tetrahedral, the inner Os atoms five-co-ordinate, square pyramidal. The bridging Os-N distances to the inner, five-co-ordinate osmiums are shorter than those to the outer osmiums; the terminal Os-N distances are essentially the same for both types of osmium. Compound 3 and the cation in 4 are dimers with imido bridges. The main difference between them is a significant shortening in the Os ... Os distance in the osmium(VIII) compound as a result of Os-Os single bond formation. This also results in a slight shortening of the Os-N bridge distances in 4 as compared to the neutral molecule 3.