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Title: A kinetic study of the induced aquations of some chromium (III) complexes
Author: Matts, Terrence Charles
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1970
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The original intention of the work was to garner evidence for or against the participation of a D process in the substitution reactions of Cr (III) complexes. The kinetics of the aquation Cr(H₂O)5N32+ induced by nitrous acid were studied as an adjunct to stoichiometric experiments using the same reaction, in which the possible five coordinate intermediate, Cr(H₂O)5 3+, was sought. The rapid reaction rate was found to be first-order in [complex], [H+], and [HNO2], and much faster in the presence of chloride ion. High concentrations (2 - 5M) of added anions were used in the stoichiometric experiments in an attempt to trap the intermediate as mon-acidopenta-aquochronium (III) complexes. Low concentrations of such species were, indeed, detected, but the discrimination of the postulated intermediate for the anions and against solvent water was small. The discrimination factors obtained for C1- and Br- were much lower than those reported by another worker during the spontaneous acid hydrolysis of Cr(H2O)5I2+, and indicate that, either the trans-labialising ability of the iodo-ligand was responsible for the incorporation of C1- and Br- in the previous work, or that different intermediates are involved in the spontaneous and induced equations. Though the evidence tends to favour the participation of a highly reactive intermediate in the induced reaction, the data are not totally inconsistent with an interchange mechanism. Due to the inconclusive mature of the results obtained in the competition studies, and the uncertainty that other approaches would provide definitive evidence, attention was turned to an interesting observation that was made during preliminary experiments to the stoichiometric work. Nitrous acid was found to be an efficient catalyst for the aquations of halo-aquochronium (III) complexes in acidic aqueous media. Other oxyacids have little or no effect at low pH values. The kinetics of aquation of Cr(H2O)5Br2+ in the presence of HNO2 were studies in detail. Reaction rates are first-order in [complex] and [HNO2] and are proportional to the reciprocal of [H+] at high acid concentrations. At lower acidities, limiting rate behaviour is indicated. The possibilities of an ion-pair mechanism or direct attack by a nitrosating species on the leaving group were excluded, and transient nitrito-intermediates, formed by the rapid O-nitrosation of an aquo-ligand, are thought to be involved in the catalysis mechanism. Studies on the isomers cis- and trans-Cr(NH3)4(H2O)C12+ showed that the equation of only the cis-isomer is catalysed by HNO2, indicating that a cis-halo-nitritochromium (III) intermediate is labile with respect to loss of halide ion. Possible explanations for this phenomenon are advanced. Preparative work reinforces the conclusions derived from kinetic studies, and several new nitritochromium (III) complexes are reported. The acid hydrolyses of nitritochromium (III) complexes were found to be both rapid and acid catalysed, in contrast to a previous report. The same rate law is applicable to all five complexes studied and contains terms in both [H+] and [H+]2. The rate constant independent of acid concentration appears to be very small. A mechanism involving singly and doubly protonated intermediates is suggested. The reactions are thought to proceed without chromium-oxygen bond cleavage because of their rapid nature, their low ∆H+ values, and by analogy with the formations of nitrito-complexes, which are believed to avoid Cr-O fission. Much of the kinetic work involved in these studies was carried out using the stopped-flow method. Modifications to a Gibson-Milnes stopped-flow spectrophotometer are described which allow the instrument to be used with highly acidic and other corrosive solutions.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry