The electron transfer reaction of [Ru-III(edta)(pyz)](-) with sulfite in aqueous solution

The reduction of [Ru-III(edta)(pyz)](-) (edta = ethylenediaminetetraacetate , pyz = pyrazine) with sulfite has been investigated spectrophotometrically in aqueous solution and found to be first order in both the complex and su lfite. The values of the observed rate constant depend on the pH, since it controls the spaciation of oxoanions of sulfur(IV). The effect of alkali ca tions (K+, Na+ and Li+) is attributed to triple-ion formation through an al kali cation bridging between two negatively charged reactants and facilitat ing the electron-transfer process. Kinetic data and activation parameters a re interpreted in terms of an outer-sphere electron transfer mechanism. The reaction has also been analysed using the Marcus cross-section relationshi p for outer-sphere electron transfer reactions.