Solvent and chelation effects on the photoreduction of cobalt(III)-amine complexes in aqueous-organic solvent media

The photoreduction of trans-[Co(NH3)(4)Cl-2](+), trans-[Co(en)(2)Cl-2](+), [Co(dien)Cl-3], [Co(trien)Cl-2](+), and [Co(tetren)Cl](2+), ions has been studied using a low pressure Hg vapour lamp as light source (254 nm) in aqueous-organic solvents [0-30% (v/v) MeOH or 1,4-dioxane]. Quantum yields for Co-II production by redox decomposition have been determined in all the cases, and increase considerably with the increase in concentration of MeOH or 1,4-dioxane in the binary solvent mixtures under investigation. A plot of log(quantum yield) versus the Grunwald-Winstein Parameter, Y, which is a measure of solvent ionizing power, shows that a different blend of general and specific solvent interacts with the solute. This kind of specific solvent interaction on the reactant/excited state has been analysed using multiple regression: viz. Krygowski-Fawcett and Kamlet-Taft equations. Reasons for the difference in reactivity with chelation are also discussed.