Co-ordination chemistry of higher oxidation states. Part 35. Synthetic, spectroscopic, and electrochemical studies of some mer- and fac-[OsL3X3]/[OsL3X3]+systems (L = PR3, AsR3, SbR3, or pyridine; X = Cl or Br): crystal structure of fac-[Os(PEt2Ph)3Cl3]·CH2Cl2