Specific cation effect on quenching reactions of excited tris(α,α′-diimine) ruthenium(II) and chromium(III) complexes by cyanide complexes in aqueous solutions

Specific salt effects were studied on the quenching reaction of excited [Ru(N-N)₃]²⁺ (N-N=2,2′-bipyridine(bpy), 1,10-phenanthrorine(phen)) and [Cr(bpy)₃]³⁺ by [Cr(CN)₆]^3-, [Fe(CN)₆]^3- and [Ni(CN)₄]^2- in aqueous solutions as a function of alkali metal ions which were added for adjustment of ionic strength. The quenching rate constants in [Ru(N-N)₃]²⁺-[Cr(CN)₆] ^3- and [Cr(bpy)₃]³⁺-[Cr(CN)₆] ^3- systems are changed by the cations as Li⁺>Na ⁺>K⁺≈Rb⁺≈Cs⁺. On the other hand, the rate constants in [Ru(N-N)₃]²⁺-[Fe(CN) ₆]^3- and [Ru(N-N)₃]²⁺-[Ni(CN) ₄]^2- systems, which are diffusion-controlled reactions, are not varied by the alkali metal cations. The obtained order (Li ⁺>Na⁺>K⁺≈Rb⁺≈Cs ⁺) of the quenching rate constant is quite different from salt effects, Li⁺<Na⁺<K⁺<Rb ⁺<Cs⁺, which have been obtained in the electron transfer reactions between complex anions.