Kinetics and Mechanism of Oxidation of Azide Ion by 12-Tungstocobaltate(III), [CoW12O40]5-, Ion

Abstract

Abstract The title reaction proceeds through the formation of an intermediate [CoIII W... N3]6 formed between [ComIIIW]5- and N3- and not between [CoIIIW]5 and HN3. The absorbance measurements of the intermediate at various [HN3] yield an equilibrium constant Κ = 0.012 ± 0.001 at 35° C which compares well with the value 0.011 obtained at that temperature from the kinetics data. The reaction is first-order with respect to [ComIIIW]5- and HN3. The pH dependence of kobs, the pseudo-first-order rate constant ([HN3] > [CoIIlW]5-), is consistent with the linearity of the plots between kobs-1 and [H+] with intercepts on the rate ordinate. The inverse correlation between kobs and [H+] is traced to the equilibrium HN3 ⇄ N-3 + H+ followed by [CoIIIW]5 + N3 ⇄ [CoIIIW... N3]6 andnot through the equilibria [CoIIlW]5 + HN3 ⇄ [ColIIW ...N3]6- + H+. The high reactivity of the N-3 ion is traced to the presence of an unshared pair of electrons as in the case of NH2OH and N2H4 which seems to imply an inner-sphere mechanism with the substrate binding to the oxidant via this pair of electrons. However, in view of the well-protected nature of the central CoIII atom in [ColIIW]5- ion there is difficulty in visualising the coordination of the substrate with the Co1" ion. That the electron transfer is outer-sphere is substantiated by excellent agreement between the experimental rate of electron-transfer, 0.305 dm3 mol-1 s-1 , and the one (0.339 dm3 mol-1 s-1) calculated by the application of Marcus equations.