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The coefficient 2 in eq. 16 is necessary because both the forward and reverse reactions of eq. 10 contribute to the observed electron transfer. Since the system is at equilibrium, the rates of these two reactions are equal. Consequently, the forward reaction of eq. 10 is only responsible for half the observed electron transfers.19 Substitution of k% = 12.1 ± 1.6 M~l sec.-1 and &e = 57.6 ± 2 M~l sec.-1 gives k% = 33.4 ± 2.6 M~l sec.-1 at 25.0°and ionic strength 3.0 .A comparison of the magnitudes of these rate constants shows that the electron exchange between Fe2+ and FeCl2 + proceeds mainly by a chlorine atom transfer mechanism (provided, as seems reasonable, the third-order term involving Fe2+, Fe3+, and chloride may be neglected).The chlorine atom transfer presumably occurs in a chloride-bridged, inner-sphere activated complex, while the electron transfer may occur in either an outersphere activated complex or in a water-bridged, innersphere activated complex. It is of interest that reaction via the chloride-bridged path is only about three (19) J. Silverman, Ph.D. Thesis, Columbia University, New York, N. Y., 1951. times as rapid as reaction via the outer-sphere or waterbridged paths. This small difference, and indeed the relatively small effect of chloride on the iron(II)-iron-(III) exchange, contrasts markedly with its effect on the iron(II)-chromium(III) and chromium(II)-chromium(III) reactions. In the latter systems, chloride bridging increases the rates of the reactions by factors of 104 and more than 2 X 106 7, respectively.18 Moreover, the chromium (I I)-catalyzed dissociation of Cr-Cl2 + is slower by a factor of more than 104 than the Cr2+-CrCl2+ exchange.20 The difference in the chloride effects cannot be ascribed to any difference in the mechanisms of the reactions since these studies show that it is very likely that the chloride-catalyzed iron (Il)-iron(III) exchange, like the chloride-catayzed chromium(II)-iron(III) and chromium(II)-chromium(III) reactions, proceeds via an inner-sphere activated complex.

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