"REACTIONS OF CHROMIUM (VI) AND COBALT (III) COMPLEXES IN AQUEOUS SOLUTIONS"
"REACTIONS OF CHROMIUM (VI) AND COBALT (III) COMPLEXES IN AQUEOUS SOLUTIONS"
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Date
1975-08
Authors
OLATUNJI, MOBOLANLE ADEGBOYEGA
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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 Uganda 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, O-Crl, 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 than 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 outersphere
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.
Description
A thesis submitted to the University of Glasgow
for the degree of Doctor of Philosophy
Keywords
REACTIONS,, CHROMIUM (VI),, COBALT (III) COMPLEXES,, AQUEOUS SOLUTIONS,