Metal-ion catalysed aquation of transition metal complexes

Mechanisms of reactions of metal complexes: Some recent developments

Activity in this area remains high, with a considerable volume of interesting work appearing. Review articles of interest include a general discussion of the chemistry of cobalt(III) complexes covering the 1981 literature,(1) and an excellent review by Tobe(2) on the base hydrolysis of metal complexes [predominantly Co(III) and Cr(III) complexes] which will be of great value in defining future research activities. Dixon and Sargeson(3) have also reviewed many aspects of the reactions of coordinated ligands on cobalt(III).

Substitution Reactions of Inert Metal Complexes—Coordination Numbers 6 and Above: Cobalt

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.

Debabrate Banerjea

Papers

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