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 kobs=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 CoIII(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.