Anation of penta-ammineaquocobalt(III) by bromide ion in aqueous media

Abstract

The aquation rate constant of the ion [Co(NH₃)₅Br]²⁺ is (3·9 ± 0·08)× 10^–6 s^–1 at 25·0 °C 1·00M ionic strength, and 0·0100M-HClO₄. The anation rate constant for the same conditions is (1·32 ± 0·06)× 10^–6 l mol^–1 s^–1 over the range 0·1000M⩽[Br^–]⩽ 0·750M and shows no saturation effect with increasing bromide-ion concentration. The mechanistic implication of this result is discussed. By independent measurements the concentration quotient for formation of the inner-sphere bromo-complex has been found to be 0·283 ± 0·030 l mol^–1 for the same conditions. Attempts to determine ion-pairing constants of Br^– with [Co(NH₃)₅H₂O]³⁺ and [Co(NH₃)₅Br]²⁺ failed.