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+ approximate toRb(+) 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+ approximate toRb(+) approximate toCs(+)) 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. (C) 2002 Elsevier Science B.V. All rights reserved.