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

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 k_obs = 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.