COORDINATION CHEMISTRY OF HIGHER OXIDATION-STATES .38. SYNTHESIS, SPECTROSCOPIC AND ELECTROCHEMICAL STUDIES OF SOME TRANS-DIHALOGENOOSMIUM COMPLEXES - CRYSTAL-STRUCTURE OF TRANS-[OS(PME3)4CL2]BF4

The complexes trans-[Os(PMe3)4X2] (X = Cl or Br) have been obtained from [Os(PPh3)3X2] and PMe3, and trans-[OsL4X2] [L = PMe2Ph, AsMe3, SbPh3 or pyridine (py)] by reduction of appropriate osmium(III) complexes in the presence of L. The complexes cis-[Os(PR3)4X2] are formed by isomerisation of the trans analogues in chlorinated solvents, and in other ways. Air oxidation of the osmium(II) complexes gives trans-[OsL4X2]BF4 (L = PMe3 or AsMe3), but formation of trans-[OsL4X2]BF4 (L = PMe2Ph, SbPh3 or py) and cis-[Os(PMe2Ph)4Cl2]BF4 requires HNO3 as oxidant. Use of concentrated HNO3 gives trans-[OsL4X2]2+ (L = PMe3, AMe2Ph or AsMe3) in solution, but these have not been isolated. The complexes have been characterised by IR, UV-VIS, and NMR spectroscopies, and the effect of stereochemistry and L and X upon the Os(II)-Os(III) and Os(III)-Os(IV) redox potentials probed by cyclic voltammetry. The crystal structure of trans-[Os(PMe3)4Cl]BF4 has been determined: orthorhombic, space group Fddd, a = 8.104(4), b = 32.195(11), c = 38.540(9) angstrom, and Z = 16. The cation has Os-P 2.419(5) and 2.398(5) angstrom and Os-Cl 2.352(4) angstrom, and shows deviations of the OsP4 unit from planarity due to steric interactions. There is no evidence that mer-[IrL3Cl3] (L = PEtPh2, AsMe2Ph, SbPh3 or SME2) can be oxidised either chemically or electrochemically to stable iridium (IV) cations.