Kinetics and mechanism of the interaction of iminodiacetate with cis-tetraamminediaquacobalt(III) ion in acidic media.

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Kinetics of the reaction of iminodiacetate (Hida−) with cis-tetraamminediaquacobalt(III) have been studied spectrophotometrically at pH 3.0–4.15, [iminodiacetate]T=0.03–0.4. mol dm−3, I=0.5 mol dm−3 (NaClO4), in the temperature range 30–50 °C. The reaction is governed by the rate law: rate={k−1+k1[iminodiacetate]T}[complex], where k−1 and k1 denote the reverse aquation and anation rate constants respectively. At 40 °C the k−1 values increase with pH and show a limiting tendency at pH≈4. The pH-anation rate profile indicates no reaction at pH<1.5 and in the range 3.5–3.9, k1 remains constant. The pH dependence on rate is expressed as k1=kAKAH/([H+]+KAH), where kA is the anation rate arising from the aqua complex ion and Hida− species and KAH, the dissociation constant for the iminodiacetic acid. The value of kA at 40 °C=(5±0.02)×10−5 mol−1 dm3 s−1. The activation parameters, ΔH\neweq and ΔS\neweq for the anation step are 170±2 kJ mol−1 and 239±3 JK−1mol−1. The reaction is believed to operate through the initial complexation at the carboxylate end followed by a fast intramolecular proton transfer with simultaneous ring closure to form a chelated product. The complexation probably takes place through a dissociative interchange (Id) mechanism.
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