A series of complexes of general formula [OsX₃L₃] and [OsCl₃L₂L'] where X is a chloride or bromide and L and L' are different tertiary phosphines or arsines have been synthesised. The X-ray diffraction crystal structures of the complexes, mer-[OsCl₃(PMe₂Ph)₃], mer-[OsCl₃(PEt₂Ph)₃], mer-[OsBr₃(PPrⁿ₃)₃], mer-[OsCl₃(AsME₂Ph)₃], mer-[OsCl₃(PPrⁿ₂)₂(AsPrⁿ₃)], mer-[OsCl₃(PPrⁿ₂)₂(PEtPh₂)] and mer-[OsCl₃(P(OMe)₂Ph)₂(AsPrⁿ₃)] show them to have slightly distorted octahedral metal environments with the trans influence of the Group 15 ligands evident. The mer complexes have been studied electrochemically and all show two one-electron processes; an oxidation and a reduction process. Both couples involve the osmium metal centre. The one electron reduction step is rapidly followed by a chemical reaction resulting in the formation of an electroactive daughter product of general formula [OsCl₂L₃Y] where Y is a neutral coordinating ligand. The chemical reaction has been studied by kinetic and spectrochemical methods. The redox potentials of the mer species gives a good linear correlation with Tolmans electronic parameter for the tertiary phosphine. The electronic spectra of the compounds have been recorded and the peaks assigned to specific electronic transitions via the use of Extended Huckel Molecular Orbital Calculations. Electrochemical elucidation of these systems has shown that the redox potentials of the complexes are predominantly dependent on one ligand, namely the phosphine/arsine trans to the halide. The fac isomers also show two metal based one-electron couples at very different potentials from the analogous mer isomers. The reduction also produces an electroactive daughter product. The oxidation results in the isomerisation of the fac isomer to that of the mer isomer. The kinetic parameters of the reaction have been measured, and a mechanism is proposed for the isomerisation.