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 kobs are dependent on the inverse of the hydrogen-ion concentration, kobs=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.