SALT AND SPECIFIC CATION EFFECTS IN THE QUENCHING OF TRIPLET-STATE TETRAKIS (MU-PYROPHOSPHITE-P,P') DIPLATINATE(II) BY ACIDOPENTACYANOCOBALTATE(III) ANIONS

In the presence of moderate to high concentrations of electrolytes, th e emission of [Pt-2(pop)(4)](4-) (where pop = mu-pyrophosphite-P,P') is quenched by the complexes [Co(CN)(5)X](3-) (where X = N-3(-), I-, B r-, Cl-, but not CN-). The salt effects on the emission decay lifetime quenching rate constants between these anionic species have been stud ied in the presence of MCl, M'Cl-2, or RnNHL4-nCl (where M, M', and R represent alkali, alkaline earth metals, and alkyl respectively, n = 0 -3) and KnX (X = Cl-, Br-, NO3-, SO42-, [Co(CN)(6)](3-), n = 1-3). At 0.5 M cation concentration, second-order quenching rate constants, k(q ), are in the ''nearly diffusioncontrolled'' range, 10(7)-10(9) L mol( -1) s(-1), and k(q) decreases by an order of magnitude across the seri es of quenchers [Co(CN)(5)I](3-) > [Co(CN)(5)N-3](3-) > [Co(CN)(5)Br]( 3-) > [Co(CN)(5)Cl](3-). On the basis of a detailed study of [Co(CN)(5 )I](3-), the quenching efficiency increases with background electrolyt e concentration and the measured rate constants are in good agreement with predictions based on the Debye-Smoluchowski and Debye-Eigen equat ions for diffusion-controlled formation and dissociation in ionic solu tion of an encounter pair, together with a rate constant of 1.2 x 10(9 ) s(-1) for the quenching step. However, the analysis provides further evidence for the Olson-Simonson effect; that is, in the presence of m ultivalent electrolyte ions, the salt effects are determined by the co unterion concentration, here the cation, rather than by the ionic stre ngth. Specific cation effects are observed such that the quenching rat e constants increase in the following sequences: Li+ < Na+ < K+ < Cs+; Mg2+ < Ca2+ < Sr2+ < Ba2+; NH4+ < MeNH3+ < Me2NH2+ < Me3NH+; Et3NH+ < Et2NH2+ < EtNH3+; n-PrNH3+ < EtNH3+ < MeNH3+. For the alkali or alkal ine-earth cations the large effects seen require participation of the cation in the transition state for the quenching step; the alkylammoni um cations are also effective in this role, but the small differences in their efficiencies can be rationalized in terms of their effects on water structure.