Kinetic and mechanistic studies on the electron-transfer reactions between diaquabisethylenediaminecobalt(III) and ethylenediaminetetraacetatocobaltate(III) with promazine in acidic aqueous media

The kinetics of the electron-transfer reactions between promazine (ptz) and Co(en)₂(H₂O)₂]³⁺ in CF₃SO₃H solution (Co^III] = (2–6) × 10⁻³ m, ptz] = 2.5 × 10⁻⁴ m, H⁺] = 0.02 − 0.05 m, I = 0.1 m (H⁺, K⁺, CF₃SO ₃ ⁻ ), T = 288–308 K) and Co(edta)]⁻ in aqueous HCl (Co^III] = (1 − 4) × 10⁻³ m, ptz] = 1 × 10⁻⁴ m, H⁺] = 0.1 − 0.5 m, I = 1.0 m (H⁺, Na⁺, Cl⁻), T = 313 − 333 K) were studied under the condition of excess Co^III using u.v.–vis. spectroscopy. The reactions produce a Co^II species and a stable cationic radical. A linear dependence of the pseudo-first-order rate constant (k _obs) on Co^III] with a non-zero intercept was established for both redox processes. The rate of reaction with the Co(en)₂(H₂O)₂]³⁺ ion was found to be independent of H⁺]. In the case of the Co(edta)]⁻ ion, the k _obs dependence on H⁺] was linear and the increasing H⁺] accelerates the rate of the outer-sphere electron-transfer reaction. The activation parameters were calculated as follows: ΔH ^≠ = 105 ± 4 kJ mol⁻¹, ΔS ^≠ = 93 ± 11 J K⁻¹mol⁻¹ for Co(en)₂(H₂O)₂]³⁺; ΔH ^≠ = 67 ± 9 kJ mol⁻¹, ΔS ^≠ = − 54 ± 28 J K⁻¹mol⁻¹ for Co(edta)]⁻.