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

Autor: Rupendra Nath Banerjee, Samir Gopal Bhattacharya, Sarmistha Das, Debabrate Banerjea
Rok vydání: 1983
Předmět:
Zdroj: Transition Metal Chemistry. 8:241-243
ISSN: 1572-901X
0340-4285
DOI: 10.1007/bf00620702
Popis: 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 (kspont.) 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 kobsvs [Hg2+] plots over a wide range of [Hg2+] concentration suggests a direct electrophilic (SE2) attack of the Hg2+ on the leaving thiocyanate ligand of the substrate complex; there is CoIII-NCS bond fission without the involvement of any long-lived binuclear intermediate of the type CoIII-NCS-HgII.
Databáze: OpenAIRE
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