Kinetics and mechanism of oxidation of [RuII(pic)3]- with peroxydisulfate in aqueous medium

The oxidation of [Ru-II(pic)(3)](-) (pic- = picolinate) with peroxydisulfate (S2O82-) was studied spectrophotometrically and kinetically in aqueous solution. The rate of the reaction followed via absorbance at 466 nm (a metal-to-ligand charge transfer band of [RuII(pic)3)]-) and was found to be first-order in the Ru(II)-complex. The values of the observed rate constant (kobs) increased linearly with an increase in [S2O82-]. The rate of the reaction was found to be independent of pH in the studied range of 3.0 to 7.5. Alkali metal ions were found to have a positive influence (K+ > Na+ > Li+) on the rate of the oxidation. Kinetic data and activation parameters (Delta H-not equal = 37 +/- 1 kJ mol(-1) and Delta S-not equal = -129 +/- 3 J mol(-1) deg(-1)) are interpreted in terms of a mechanism involving outer-sphere electron transfer between the reaction partners. A detailed reaction mechanism in agreement with the spectral and kinetic data is presented and discussed with reference to that reported for the oxidation of [RuII(pic)3]- with H2O2.