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Base hydrolysis of iso-thiocyanatopentaammine complexes of cobalt(iii), chromium(iii) and rhodium(iii) in aqueous-organic mixed solvents
Published in -
Volume: 13
Issue: 1
Pages: 63 - 69
In the base hydrolysis of the title complexes in aqueous-organic solvent media (water-ethanol and water-acetone) to the corresponding hydroxo-complexes the observed rate law is in accord with the expression: [OH-]/kobs = [OH-]/k + 1/kK. The observed K values (3.5 to 26 M-1 at 25°) and the nature of their solvent dependence suggest the formation of an ion-pair in a pre-equilibrium involving the substrate complex and OH- ion, and not of a conjugate base. Rate constants (k) for the transformation of the ion-pair to the product increase with increased proportions of the organic component in the solvent. The nature of the dependence of k on different solvent parameters indicates significant associative character with OH- ion acting as the attacking nucleophile. Activation enthalpy, ΔH≠, decreases with ethanol concentration; this along with the observed large positive slopes of the log k vs 1/D plots rule out the possibility of H2 O acting as the attacking nucleophile. ΔH≠ and ΔS≠ corresponding to k in different solvent compositions for a particular complex are linearly correlated. A common mechanism appears plausible for all the complexes in which bond breaking and bond making are synchronous but their relative contributions are different giving a relatively more associative character to the cobalt(III) and rhodium (III) systems compared to the chromium(III) system. This view is supported by the magnitudes of the slopes of the log k vs 1/D, and log k vs Y plots. The dissociation of the M-NCS bond is probably solvent assisted due to solvation of the departing ligand as indicated by the linear dependence of ΔH≠ and ΔS≠ on different solvent parameters. © 1983, Taylor & Francis Group, LLC. All rights reserved.
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JournalJournal of Coordination Chemistry