Redox and Spectral Properties of the Cis and Trans Isomers of the Osmium(VI) Dioxo Complex [(bpy)₂Os(O)₂](ClO₄)₂

The cis and trans isomers of [(bpy)₂Os(O)₂](ClO₄)₂(bpy = 2,2'-bipyridine) have been prepared and characterized and their redox properties in aqueous solution investigated by using electrochemical techniques. Plots of E_1/2 vs. pH for a series of redox couples that appear involving oxidation states II—VI are revealing both in terms of the relative stabilities of the various oxidation states for each isomer and in terms of the relative stabilities of the cis and trans isomers in different oxidation states. The cis isomer undergoes bpy ligand loss in aqueous solution on a time scale of minutes by chelate ring opening followed by ligand loss. Upon reduction to Os(III) or Os(II) the trans isomer is unstable with respect to isomerization to the cis isomer. Comparisons of the redox properties of the isomeric pair give insight into the factors that dictate the relative stabilities of oxidation states and suggest possibilities for the control of the redox potentials, which play a key role in the ability of polypyridyl oxo complexes of ruthenium and osmium to act as redox catalysts.