Laser flash photolysis study of the charge-transfer photochemistry of octacyanomolybdate(V) and octacyanotungstate(V) ions
Abstract
Laser flash photolysis (at 249 nm) of [MV(CN)8]3–(M = Mo or W) in water and methanol produces an intense absorption (λmax. 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. {[MIV(CN)7(ĊN)]3–, Mx+}(Mx+= H+, Li+, Na+, K+, Cs+, NH4+, NEt4+, or Ca2+), which decays to the final product [MIV(CN)8]4–. Detailed analysis by computer-fitting of the accelerative effects of different cations gave values for the absolute decay rate constant, ko, the specific decay rate constants, kMx, and the association constants, KMx, which decrease in the sequences: Cs+ > H+ > Ca2+ > K+ > NH4+ > Na+ > Li+ > NEt4+ and NEt4+ > H+ > Ca2+ > Li+ > Na+ > NH4+ > K+ > Cs+ for kMx and KMx respectively.