Kinetics of the reduction of (ethylenediaminetetra-acetato)cobaltate(III) and related complexes by chromium(II) ions

Abstract

Rate constants of the electron-transfer reactions ofthree cobalt(III)–edta (edta = ethylenediaminetetra-acetate) complexes with Cr²⁺ have been determined by a stopped-flow spectrophotometric method at 25 °C and an ionic strength of 1.0 mol dm^–3(Na[ClO₄]). The values are (6.6 ± 1.0)× 10³ dm³ mol^–1 s^–1 for [Co(edta)]^–, (7.6 ± 0.4)× 10³ dm³ mol^–1 s^–1 for [Co(Hedta)(OH₂)], and > 2 × 10⁶ dm³ mol^–1 s^–1 for [CoCl(Hedta)]^–. A comparison with other reducing agents {Fe²⁺, [Fe(CN)₆]^4–, and Ti^III} indicates that titanium (III) reduction of the same series of cobalt(III)–edta complexes, studied by Marčec and Orhanović, proceeds via an outer-sphere mechanism.