The H+, Fe3+ and AI3+ catalysed aquation of trans-bis(Hmalonato)-bis(ethylenediamine )cobalt(III) yields cis-(aquo)(Hmalonato)bis(ethylenediamine)cobalt(III) as the major cobalt(III) product. The binuclear complexes, trans-[(en)2Co(malH)malM](n+1)+ (M = Fe3+, Al3+) are the reactive species. The equilibrium constant for formation of binuclear species and the rate constant of its aquation are reported. For the acid catalysed aquation in the range of [H+] = 0.1-0.8 mol dm-3 (I = 1.0 mol dm-3), kobs versus [H+] plot is linear indicating that the complex does not undergo significant protonation under the condition of acidity employed. The activation parameters (DH1 and DS1) for the metal ion and H+ catalysed paths are comparable to those for the trans ->cis isomerisation of trans- [Co(en)2(OH2)malH]2+. This is consistent with the fact that trans ->cis isomerisation in the present situation actually occurs during substitution reaction
