The kinetics of intramolecular substitution reactions of some binuclear complexes of cobalt in perchlorate media

Abstract

The kinetics of the reaction of the aquo-chloro complex (NH₃)₄(H₂O)Co·NH₂·Co(Cl)(NH₃)₄⁴⁺, in which a hydroxo-bridge is formed, and the corresponding reactions of the aquo-bromo- and diaquo-complexes, (NH₃)₄(H₂O)Co·NH₂·Co(Br)(NH₃)₄⁴⁺ and (NH₃)₄(H₂O)Co·NH₂·Co(H₂O)(NH₃)₄⁵⁺: [graphic omitted] have been studied in the range 15–45° in perchlorate media (µ 0·1 M and 2·0M). The reactions are first-order in complex, the hydrogen-ion dependence being of the form k_obs=k₁+k₂^′[H⁺]^–1 over the pH range 0–3. The rate constant, k₁, corresponds to the reaction as in (i), and k₂′ in all probabilty to the reaction of the related hydroxo-complex as in (ii): [graphic omitted]. At 25°, rate constants k₁ are 10¹—10³ times greater than for the aquation of mononuclear complexes of general formula Co^III(NH₃)₅Xⁿ⁺. Enthalpies of activation for k₁ are more favourable (ca. 8 kcal.mole^–1), and entropies of activation less favourable (10–20 e.u.) than for the corresponding mononuclear complexes. Values for k₂′ are very much dependent on the ionic strength, which has been varied over the range 0·001–2·0M for the aquo-chloro-complex. Possible mechanisms are considered.