Outer-sphere electron transfer in methylene chloride: Concentration, salt, and temperature dependences of the oxidation of beta-Re(2)X(4)(cis-1,2-bis(diphenylphosphino)ethylene)(2) (X=Cl, Br) by [Co(dimethylglyoximate)(3)(BF)(2)]BF4 and the oxidation of Re2Br4(PMe(2)Ph)(4) by [Co(1,2-cyclohexanedione dioximate)(3)(BBu)(2)]BF4

The kinetics of the oxidation of beta-Re(2)X(4)(cis-1,2-bis (diphenylphosphino) ethylene)(2) (X = Cl, Br) by the cobalt clathtrochelate [Co(dimethylglyoximate)(3)(BF)(2)]BF4 and the oxidation of Re2Br4(PMe(2)Ph)(4) by the cobalt clathrochelate [Co(1,2-cyclohexanedione dioximate)(3)(BBu)(2)]BF4 have been studied by the stopped-flow method as a function of temperature (-85 to -19 degrees C), added Bu(4)NBF(4) (0-0.100 M), and reactant concentration in the low dielectric solvent methylene chloride. For each reaction, similar to 100 different conditions were studied. The observed rate constants were well fit by a mechanism involving separate paths for free ion and the ion-paired Co(III) oxidant. The analysis yielded values for Delta H double dagger and Delta S double dagger for each path of each reaction and consistent Delta H degrees and Delta S degrees values for the ion-pairing of the cationic reactant and the electrolyte, In addition, temperature-dependent electrochemical measurements in 0.10 M Bu(4)NBF(4) yielded Delta H degrees and Delta S degrees for the electron transfer process. This is the first measurement of the homogeneous electron transfer reactivity of the dirhenium complexes, and they showed the expected high reactivity. The most notable result is a very high inhibition (ca. 700-fold) by added salt of only the [Co(dmg)(3)(BF)(2)]BF4 reactions. We attribute this to a change of rate-controlling step, for the ion-paired path, to one involving anion migration. This appears only to occur when the magnitude of ion-pairing free energy is significantly greater than the magnitude of the free energy change for the electron transfer process.