Kinetics of reactions of bis(isothiocyanato)bis(dimethylglyoximato)cobaltate(III) in presence and absence of Hg²⁺ ion in acid perchlorate media

Summary

At elevated temperatures (⩾ 70 °C) the title complex aquates significantly in aqueous acid media, releasing one thiocyanate in the first step, which exhibits no acid dependence. For this spontaneous aquation, the rate constant (k_spont.) at 25 °C, ΔH^≠ and ΔS^≠ are 1.2×10⁻⁸s⁻¹, 132 kJ mol⁻¹ and 59 JK⁻¹mol⁻¹, respectively. This first step is followed by a slower second step in which, the second thiocyanate is released as the Co(dmgH)₂ moiety is cleaved. The rate of release of thiocyanate is enhanced considerably by addition of Hg₂₊ and the complex changes smoothly into the corresponding di(aqua) product, [Co(dmgH)₂(OH₂)₂ ⁺, without any complicating side reactions. Both the steps of the process are linearly dependent on [Hg²⁺]. For the catalysed reaction, the second-order rate constant at 25 °C, ΔH^≠ and ΔS^≠ for the first and second steps are 2.85 M⁻¹s⁻¹, 89 kJ mol⁻¹, 60 JK⁻¹mol⁻¹; and 1.48 M⁻¹s⁻¹, 63 kJ mol⁻¹, −30 JK⁻¹mol⁻¹, respectively. The observed linearity of the k_obs vs [Hg²⁺] plots over a wide range of [Hg²⁺] concentration suggests a direct electrophilic (S_E2) attack of the Hg²⁺ on the leaving thiocyanate ligand of the substrate complex; there is Co^III-NCS bond fission without the involvement of any long-lived binuclear intermediate of the type Co^III-NCS-Hg^II.