The kinetics of the anation reaction of aquopentaamminecobalt(III) by malonate in acidic aqueous solution

The Co(NH₃)₅OH₂³⁺ ion reacts with malonate to form Co(NH₃)₅O₂CCH₂CO₂H²⁺ or Co(NH₃)₅O₂CCH₂CO₂⁺, depending on the pH of the reaction solution. The kinetics of this anation reaction have been studied as a function of [H⁺] for the acidity range 1.5 ≤ pH ≤ 6.0 in the temperature range of 60 to 80°C, the [total malonate] ≤ 0.5 M, and the ionic strength 1.0M. The anation by malonic acid follows second-order kinetics, the rate constant being 8.0 × 10^?5 M^?1·sec^?1 at 70°C, and the anations by bimalonate (Q₁, k₁) and malonate ion (Q₂, k₂) are consistent with an I_d mechanism. Typical values at 70°C for the ion pair formation constants are Q₁ = 1.3, Q₂ = 5.4M^?1; and for the interchange rate constants k₁ = 5.3 × 10^?4; k₂ = 7.3 × 10^?4 sec^?1. The activation parameters for the various rate constants are reported and the results discussed with reference to previously reported data for similar systems.