Kinetics And Mechanisms Of Oxidation Of Some Transition Metal Complexes By N-Bromosuccinimide In Aqueous Solution

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Nasma Dafalla, Eljack
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The present work is an spectrophotometric investigation on the kinetics and mechanisms of oxidation of some transition metal complexes (Aquaethylenediaminetetraacetatochromate(III) [CrIII(EDTA)(H2O)]- (A), cis-diaquabis(oxalato)chromate(III) cis-[CrIII(ox)2(H2O)2]- (B), cis-diaquabis(1,10-phenanthroline)chromium(III) cis-[CrIII(phen)2(H2O)2]3+ (C) and Aquaethylenediamine-tetraacetatocobaltate(II) [CoII(EDTA)(H2O)]2- (D), by N-bromosuccinimide (NBS) in aqueous solution at 25.0 oC. The oxidation of chromium(III) complexes exhibited biphasic kinetic behavior that can be accounted for by a consecutive two-step reaction, and the intermediates, chromium(VI,V) were detected by EPR spectroscopy. The observed rate constants, kf (kfast) and ks (kslow), were determined by computer programmed fitting using Origin7.0, experimental data were found to fit to a biexponential equation (Y =Y0 + A1e-x/t1 + A2e-x/t2). Both rate constants showed first-order dependence on [complex] and [NBS] and increased with increasing pH. The values of kf and ks were also calculated from extrapolation lines of absorbance for comparative purpose. The oxidation of aquaethylenediaminetetraacetatochromate(III) and cis-diaquabis(oxalato)-chromium(III) by N-bromosuccinimide (NBS) to yield chromium(VI) has been studied over the pH 6.0 –7.0, in the two cases, the first step is a fast electron transfer step which leading to chromium(IV) and the second step is a slow electron transfer step leading to chromium(V), followed by another electron transfer step giving chromium(VI) and the last was so fast as no EPR signal for chromium(V) was detected. The oxidation of cis-diaquabis(1,10-phenanthroline)chromium(III) by N-bromosuccinimide (NBS) to yield cis-dioxobis(1,10-phenanthroline)chromium(V) has been studied over the pH 1.57– 3.56 and 5.68 – 6.68 ranges. The reaction exhibited biphasic kinetics at pH < 4.0 and a simple first order at the pH > 4.0. In the low pH range, the reaction proceeds by two consecutive steps; the first faster step corresponds to the oxidation of chromium (III) to chromium (IV), and the second slower one to the oxidation of chromium (IV) to chromium (V), the final product of the reaction as shown by EPR signal. Each step exhibits a first-order dependence on [Cr(III)]. The rate laws which are consistent with the proposed mechanism were postulated for the above chromium(III) complexes reactions. The oxidation of aquaethylenediaminetetraacetatocobaltate(II) [Co(EDTA)(H2O)]-2 by N-bromosuccinimide (NBS) in aqueous solution has been studied over the pH 5.88 - 7.02 range. The reaction is first order with respect to the complex and the oxidant, and inverse first order on [H+] and it obeys the following rate law: Rate= k2K2K3[CoII(EDTA)(H2O)-2]T [NBS] / ([H+] +K2) It is proposed that the electron transfer proceeds via an inner-sphere mechanism due to the formation of the initial cobalt(III) which is converted slowly to the final product. The values for the rate constant of electron transfer and the equilibrium constant for all these reactions were calculated, and a mechanism which is consistent with experimental rate law is proposed for each system. The effect of ionic strength on the reaction rate was investigated for the four systems at constant reaction condition. The results showed that the rate is independent of ionic strength; this behavior is expected for a reaction in which one reactant is uncharged.
Kinetics And Mechanisms Of Oxidation Of Some Transition Metal Complexes By N-Bromosuccinimide In Aqueous Solution