Electrochemical Studies of Coordination Complexes of Osmium

Abstract

Several electrochemical techniques have been employed in the study of the transition metal complexes [OsCl₆__npy_n]^z+, n = 0–6., py = pyridine. Cyclic voltammetry has confirmed that [OsCl₆]²- undergoes an EC process at room temperature in CH₂Cl₂/py to form [OsCl₅py]²-. The rate determining step is chloride loss from [OsCl₆]³- and further investigations of other hexahalide osmate(III) species shows the rate of halide loss increases [OsCl₆]^3-<[OsBr₆]^3-<[OsI₆]^3-. Bulk electrogeneration experiments at precise potentials and temperatures result in formation of all members of the series [OsCl₆__npy_n]^z+, n = 0–6, except for the fac-isomer of the trisubstituted species. Each member of the series exhibits a rich redox chemistry and the voltammetric data for 22 closely related couples is presented. The experimental E_1/2 values are compared with predicted half-wave potentials from two models with satisfactory agreement for the Os(IV)/Os(III) and Os(V)/Os(IV) couples. We conclude that the measured E_1/2 values for the Os(lII)/Os(II) couple is dependent on n, the isomeric form and the geometric arrangement of the pyridine ligands.