Reactions of Chromium (VI) and Cobalt (III) Complexes in Aqueous Solutions

Olatunji, Mobolanle Adegboyega (1975) Reactions of Chromium (VI) and Cobalt (III) Complexes in Aqueous Solutions. PhD thesis, University of Glasgow.

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Abstract

The stopped-flow apparatus has been used to study the reactions of chromium (VI) and 12-tungsto-cobaltate (III) anions with sulphur-containing organic ligands in aqueous perchlorate solutions. The oxidation of thiourea and its N-Substituted derivatives (L) by chromium(VI), HCrO4-, proceeds via the formation of 1:1 complexes, O3CrL, which are considered to be sulphur bonded. These intermediates, formed within the time of mixing on the stopped-flow scale have been characterised, and from changes in the initial absorbance, the spectrum and thermodynamic parameters associated with complex formation have been determined. The kinetics of the electron-transfer reactions to yield chromium (III) complexes as products have been investigated, with the principal pathway involving a second-order dependence on the ligand. The significance of this observation is discussed. The ion HCrO4- also oxidises thiols such as penicillamine, glutathione and 2-mercaptoethylamine via the formation of intermediate sulphur-bonded esters. Unlike the complexes formed with the thioureas, the rates of formation of these esters are sufficiently slow for kinetic measurements to be made using the stopped-flow method. The rate constants for the acid-catalysed formation pathway were found to be smaller then those hitherto reported for similar reactions involving this oxidant. The significance of these values is discussed and comparison made with other values reported. Second-order dependences of the redox rates on ligand concentrations are also described. Finally, in order to attempt an evaluation of outer-sphere systems, the reduction of the large 12-tungsto-cobaltate(III) anion by thiourea and its N-substituted derivatives has also been investigated using the stopped-flow technique. No complex formation was detected but some deviation of the rate from first-order ligand dependence at higher ligand concentrations and low temperatures is interpreted in terms of the formation of outer-sphere complexes. Possible mechanisms of the reactions are discussed.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Inorganic chemistry
Date of Award: 1975
Depositing User: Enlighten Team
Unique ID: glathesis:1975-78698
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 15:01
Last Modified: 30 Jan 2020 15:01
URI: http://theses.gla.ac.uk/id/eprint/78698

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