Kinetics of Acid, Iron{III) & Aluminium(III) Catalysed Aquation of trans-Bis(Hmalonato)bis(ethylenediamine)cobalt(III) Ion

Abstract

The H⁺, Fe³⁺ and AI³⁺ catalysed aquation of <em>trans-</em>bis(Hmalonato)-bis(ethylenediamine )cobalt(III) yields <em>cis-</em>(aquo)(Hmalonato)bis(ethylenediamine)cobalt(III) as the major cobalt(III) product. The binuclear complexes, <em>trans-</em>[(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 (<em>I</em> = 1.0 mol dm^-3), <em>k</em>_obs versus [H⁺] plot is linear indicating that the complex does not undergo significant protonation under the condition of acidity employed. The activation parameters (D<em>H</em>¹ and D<em>S</em>¹) for the metal ion and H⁺ catalysed paths are comparable to those for the <em>trans </em>-><em>cis </em>isomerisation of <em>trans- </em>[Co(en)₂(OH₂)malH]²⁺. This is consistent with the fact that <em>trans </em>-><em>cis</em> isomerisation in the present situation actually occurs during substitution reaction.