Co-ordination chemistry of higher oxidation states. Part 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, PMe2Ph 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 OsII–OsIII and OsIII–OsIV redox potentials probed by cyclic voltammetry. The crystal structure of trans-[Os(PMe3)4Cl2]BF4 has been determined: orthorhombic, space group Fddd, a= 8.104(4), b= 32.195(11), c= 38.540(9)Å, and Z= 16. The cation has Os–P 2.419(5) and 2.398(5)Å and Os–Cl 2.352(4)Å, 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.