The kinetics of some outer-sphere reactions of europium(II) with cobalt(III) ammine complexes

Abstract

The kinetics of the reactions of europium(II) with hexa-amminecobalt(III), aquopenta-amminecobalt(III), and µ-amido-deca-amminedicobalt(III) have been studied in aqueous perchloric acid solutions of ionic strength µ 0·40M(using LiClO₄) at 20–50°. All three reactions are independent of hydrogen-ion concentration over the range [H⁺]= 0·03 to 0·36M, which is consistent with the reactions proceeding by an outer-sphere mechanism. In each case the kinetics conform to a rate equation of the form: Rate =k_obs[Eu²⁺][Complex]. At 25° rate constants and activation parameters are: k_obs= 1·7 × 10^–3 I. mole^–1 sec.^–1, ΔH^‡= 8·8 ± 0·5 kcal. mole^–1, and ΔS^‡=–42 ± 1·6 e.u. for Co(NH₃)₆³⁺; k_obs= 7·4 × 10^–2 I. mole^–1 sec.^–1, ΔH^‡= 9·3 ± 0·5 kcal. mole^–1, and ΔS^‡=–32·5 ± 1·6 e.u. for Co(NH₃)₅(H₂O)³⁺; and k_obs= 5·8 × 10^–2 I. mole^–1 sec.^–1, ΔH^‡= 9·6 ± 0·5 kcal. mole^–1, and ΔS^‡=–32 ± 1·6 e.u. for (NH₃)₅Co·NH₂·Co(NH₃)₅⁵⁺.

The rate constant for the reaction of chromium(II) with (NH₃)₅Co·NH₂·Co(NH₃)₅⁵⁺ at 25° and µ 0·40M is 3·07 × 10^–3 I. mole^–1 sec.^–1. Rate constants and activation parameters for the reactions of Eu²⁺, V²⁺, and Cr²⁺ with Co(NH₃)₆³⁺, Co(NH₃)₅(H₂O)³⁺, and (NH₃)₅Co·NH₂·Co(NH₃)₅⁵⁺ are compared.