Further studies of the kinetics of the reduction of superoxo-bridged dicobalt complexes with iron(II) in aqueous perchloric acid solutions

Abstract

The reactions of the superoxo-complexes (NH₃)₅Co·O₂·Co(NH₃)₅⁵⁺, (NH₃)₄Co·µ(NH₂,O₂)·Co(NH₃)₄⁴⁺, and (en)₂Co·µ(NH₂,O₂)·Co(en)₂⁴⁺ with iron(II) have been studied in aqueous perchloric acid solutions with use of LiClO₄ to adjust the ionic strength to µ 2·0M. The initial products are predominantly the peroxo-complexes with charges 4+, 3+, and 3+ respectively, where the single-bridged peroxo-complex (NH₃)₅Co·O₂·Co(NH₃)₅⁴⁺ rapidly decomposes to give Co²⁺ ions and oxygen. Second-order rate constants k_obs are dependent on the inverse of the hydrogen-ion concentration, k_obs=k₁[H⁺]^–1+k₂. At 9·0°, k₁ values are 2·7 × 10^–4, 3·4, and 0·46 sec.^–1 and k₂ values 2·84 × 10^–2, 25·2, and 21·1 l.mole^–1 sec.^–1 respectively. In the reactions of (NH₃)₅Co·O₂·Co(NH₃)₅⁵⁺ and (NH₃)₄Co·µ(NH₂,O₂)·Co(NH₃)₄⁴⁺ a comparison is made with previous data with NaClO₄ to adjust the ionic strength.

In the corresponding reaction of the tri-bridged complex (NH₃)₃Co·µ(NH₂,OH,O₂)·Co(NH₃)₃³⁺ up to three moles of iron(II) are consumed per mole of complex depending on the ratio of reactions. Attention was confined to the initial one-equivalent reduction of the superoxo- to peroxo-complex. There is no contribution from k₁, and in both HClO₄–NaClO₄ and HClO₄–LiClO₄ solutions a direct hydrogen-ion-dependent term in k₃ is observed. At 9·0° in HClO₄–LiClO₄, µ 2·0M, k₂= 1·52 l. mole^–1 sec.^–1 and k₃= 0·115 l.² mole^–2 sec.^–1. Sulphate and chloride have only a mild catalytic effect, ca. 10² times less than that observed for the reaction of iron(II) with (NH₃)₅Co·O₂·Co(NH₃)₅⁵⁺.

Enthalpies of activation corresponding to k₂ fall within the range 6·4 to 7·0 kcal. mole^–1 for all four reactions, there being no apparent trend in these values. The reactivity pattern for k₂ is determined therefore by variations in ΔS₂^‡ where steric effects are believed to be important in determining such variations. The effectiveness of terms in k₁ and k₃, and of chloride and sulphate ions is discussed.