Kinetics of the acid and base hydrolysis of αβR-and αβS-isothiocyanatotetraethylenepentaminecobalt(III) cations

Abstract

The kinetics of the acid hydrolysis of αβS-Co(Htetren)NCS²⁺ in perchloric and acetic acid solutions was investigated, spectrophotometrically and by SCN^– release, while the base hydrolysis of the same complex and its diastereoisomer, αβR, was studied by the spectrophotometric and pH-stat methods. The acid hydrolysis follows a rate expression for opposed first- and second-order reactions. In perchloric acid solutions the pseudo-first-order rate constant for the forward reaction was found to have an inverse hydrogen-ion dependence of the form k_a=k₁′/[H⁺] while the second-order rate constant for the reverse reaction showed a similar inverse dependence on [H⁺]; k_b=k_–1′/[H⁺]. The equilibrium constant for the reaction was independent of [H⁺]. The reaction was studied over the range 0·002–0·10M-HClO₄. At 90·00 °C and 0·0106M-HClO₄(µ= 1·00M, NaClO₄), k_a=(4·5 ± 0·3)× 10^–6 s^–1; k_b= 1·82 × 10^–2 l mol^–1 s^–1; and K_a= 0·247 × 10^–3 mol l^–1. In acetic acid solutions and over the acetic acid concentration range 0·10–1·0M the pseudo-first-order rate constant had a hydrogen-ion dependence of the form k_a′, =k₂′, +k₃′/[H⁺], while the second-order rate constant for the reverse reaction as well as the equilibrium constant showed a complex dependence on [H⁺], [AcO^–], or both. At 90·00 °C and 1·06M-acetic acid k_a′=[7·2 ± 0·1]× 10^–6 s^–1, k_b′= 3·8 ± 10^–3 l mol^–1 s^–1, and K_a′= 1·871 × 10^–3 mol l^–1. The acid hydrolyses were studied at 90·00 ± 0·03, 95·00 ± 0·03, and 99·90 ± 0·07 °C. The base hydrolyses were studied at 15·00 ± 0·01, 20·00 ± 0·02, and 25 ± 0·02 °C, over the pH range 9·22–10·37. The reactions followed a second-order rate law. At 25·00 °C and 5·19 × 10^–5M-NaOH (µ= 1·0M, NaClO₄) the pseudo-first-order rate constant for the base hydrolysis of αβR-Co(Htetren) NCS²⁺ the pseudo-first-order rate constant has the value k_c=(5·73 × 0·10)± 10^–3 s^–1. For the base hydrolysis of αβR-Co(Htetren)NCS²⁺ the pseudo-first-order rate constant has the value k_d=(5·50 × 0·02)± 10^–3 s^–1 at 25·00 °C and 6·00 × 10^–5M-NaOH. Values of ΔH‡ and ΔS‡ for the various kinetic parameters are reported. Mechanisms for both the acid and base hydrolyses are suggested.