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 ka=k1′/[H+] while the second-order rate constant for the reverse reaction showed a similar inverse dependence on [H+]; kb=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), ka=(4·5 ± 0·3)× 10–6 s–1; kb= 1·82 × 10–2 l mol–1 s–1; and Ka= 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 ka′, =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 ka′=[7·2 ± 0·1]× 10–6 s–1, kb′= 3·8 ± 10–3 l mol–1 s–1, and Ka′= 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 kc=(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 kd=(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.