Kinetics and mechanism of anation of Aquopentaamminecobalt(III) by carboxylate ligands: Mechanism of formation of Succinatopentaammine cobalt(III) complex

Dasha, Anadi C. ; Dasha, Madhu S. (1980) Kinetics and mechanism of anation of Aquopentaamminecobalt(III) by carboxylate ligands: Mechanism of formation of Succinatopentaammine cobalt(III) complex Journal of Coordination Chemistry, 10 (1-2). pp. 79-84. ISSN 0095-8972

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Official URL: http://www.tandfonline.com/doi/abs/10.1080/0095897...

Related URL: http://dx.doi.org/10.1080/00958978008079854

Abstract

The kinetics of anation of aquopentaamminecobalt(III) by succinic acid, H-succinate and succinate dianion have been investigated at 50° -65.4°C and I=0.3 M. The observed pseudo-first-order rate constant is given by kobs = where k 0 is the second order rate constant of anation of (NH3)5 CoOH2 3+ by succinic acid; k 1, Q 1 and k 2, Q 2 are the anation rate constants and the stability constants of the H-succinate and succinate ion-pairs of (NH3)5 CoOH2 3+ respectively. At 60°C, the values of the rate constants and the associated δH # (kcal mol−1) and δS # (cal deg−1 mol−1) for the k 0, k 1 and k 2 paths are (2.10 ±0.04)×10−5 M−1 sec−1, 23.7 ±0.5, -9 ±2; (5.2 ±0.5)×10−4 sec−1, 25.1 ±1.2, 1 ±4 and (7.6 ±0.3)×10−5 sec−1, 20.3 ±2.8, -16 ±9 respectively. Values of Q 1, and Q 2 at 50-65.4°C (I=0.3 M) are 1.0 ±0.1 and 27 ±2 M−1 respectively. The reactivity sequence of the ion-pairs, (k 1/k 2 5) is attributed to the transformation of {(NH3)5 CoOH2, succ}+ to its less reactive hydroxo form {(NH3)5 Co OH, Hsucc}+ in a last internal acid base equilibrium step. The value of k 0 in 90% D2 O is slightly higher than in water (k 0 H2 O/k 0 D2 O=0.95). The rate limiting water dissociation mechanism is believed to operate in the anation of (NH3)5 CoOH2 3+ by succinic acid and its anions.

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