Effect of solvent medium on the photoreduction of cobalt(III) complexes

The photochemical behaviour at 365 nm of [Co(NH<SUB>3</SUB>)<SUB>5</SUB>(NO<SUB>2</SUB>)]<SUP>2+</SUP> and [Co(NH<SUB>3</SUB>)<SUB>5</SUB>(N<SUB>3</SUB>)]<SUP>2+</SUP> has been investigated in aqueous media containing added hydioxylic solvents to examine the role of solvent viscosity in determining the extent of radical breakdown, Φ(redox)(and also, in the case of the nitro-complex, linkage isomerization). In agreement with earlier findings, Φ(redox) decreases by &gt;30% on addition of glycerol or ethylene glycol, but to interpret this effect as purely due to changing viscosity is an oversimplification, for the addition of cyclohexanol to give the same relative viscosity leaves Φ(redox) unchanged. Evidently a more specific role exists for the added solvent. The effect of adding synthetic polymers to give high viscosities again leaves Φ(redox) unaffected, but this may simply reflect the heterogeneous nature of polymer solutions