Kinetics and mechanism of acid and base hydrolysis of cis-(bioxalato) (imidazole)bis(ethylenediamine)cobalt (III) ion: A study of the role of the coordinated imidazole and its anion

Abstract

The aquation and base hydrolysis of <em>cis</em>-[(en)₂(imH)CoC₂O₄H]² + (imH = imidazole) have been investigated in 1.0 mol dm^-3 (CIO<img src='http://www.niscair.res.in/jinfo/Subscript4.gif' border=0>), ionic strength. The aquation of the complex at [H⁺] = 0.50-1.0 mol dm^-3 (60°C) proceeds via spontaneous and H⁺ catalysed paths, <em>k</em>_obs = <em>k</em> + <em>K</em>_H[H⁺], with <em>k</em><em> =</em> (1.2± 0.2) x 10^-6 s^-1 and <em>k</em>_H= (3.2± 0.4)10^-6 dm³ mol^-1s^-1. In the range [OH^-]_T= 0.010-1.0 mol dm^-3 and 15-35°C, the complex exists in doubly deprotonated form, <em>cis</em>-[(en)₂Co(im)C₂O₄] and the pseudo-first order rate constant fits the relationship, <em>k</em>_obs, = <em>k</em>₀ + k₁[OH^-] + <em>k</em>₂[OH^-]². The rate constants (<em>k</em>₀, <em>k</em>₁, and <em>k</em> and the associated activation parameters (<img src='http://www.niscair.res.in/jinfo/delta.gif' border=0><em>H</em>^≠ and <img src='http://www.niscair.res.in/jinfo/delta.gif' border=0><em>S</em>^≠<em>) </em>arc reported. Comparison of <em>k</em>₀ and <em>k</em> show that imidazolate anion (im^-) has stronger σ-bonded labilisation of Co - O bond relative to imidazole. Base hydrolysis in 50% medium results in rate acceleration relative to aqueous medium, and <em>k</em>_obs = <em>k</em>₁[OH^-l]_T is obeyed at [OH^-]_T = 0.050-0.70 mol dm^-3 (30°C).