Laser flash photolysis study of the charge-transfer photochemistry of octacyanomolybdate(V) and octacyanotungstate(V) ions

Abstract

Laser flash photolysis (at 249 nm) of [M^V(CN)₈]^3–(M = Mo or W) in water and methanol produces an intense absorption (λ_max. 410 nm) decaying by first-order kinetics with a long lifetime (τ∼ 5 ms in H₂O at 293 K). The transient lifetime is profoundly shortened (by orders of magnitude) on adding hydrogen ion and/or electrolytes, in a manner suggesting cation-, but not anion-, specificity, and implying ion pairing in the accelerated deactivation process. The transient is assigned to an ion-paired radical-pair configuration, viz. {[M^IV(CN)₇(ĊN)]^3–, M_x⁺}(M_x⁺= H⁺, Li⁺, Na⁺, K⁺, Cs⁺, NH₄⁺, NEt₄⁺, or Ca²⁺), which decays to the final product [M^IV(CN)₈]^4–. Detailed analysis by computer-fitting of the accelerative effects of different cations gave values for the absolute decay rate constant, k_o, the specific decay rate constants, k_Mx, and the association constants, K_Mx, which decrease in the sequences: Cs₊ > H⁺ > Ca²⁺ > K⁺ > NH₄⁺ > Na⁺ > Li⁺ > NEt₄⁺ and NEt₄⁺ > H⁺ > Ca²⁺ > Li⁺ > Na⁺ > NH₄⁺ > K⁺ > Cs⁺ for k_Mx and K_Mx respectively.