Kinetic Studies of the Electron Transfer Reactions in Iron(II) and Iron(III) Systems. XII. The Ratios of the Reaction Rates of the Outer-sphere vs. Inner-sphere Electron Transfer Mechanisms in the Presence of X− (X−=Cl−, SCN−, and N3−) in Aqueous Solutions, as Judged from the Kinetics of the Dissociation Reactions of FeX2+

The electron transfer reactions between Fe2+ and FeX2+ (X−=Cl−, SCN−, or N3−) may take place through the inner- or outer-sphere mechanism or through both in parallel. The process of the outer-sphere mechanism is just equivalent to the process of dissociation of FeX2+ catalyzed by Fe2+ in appearance. When the overall rate constant of the electron transfer reaction and the rate constant of Fe2+-assisted dissociation reaction of FeX2+ are denoted by kE and ko respectively, the fraction r of the outer-sphere mechanism to the total is given in terms of r=2ko⁄kE. Thus, the r values were determined with the results that r(Cl−)=44% at I=1.5 M (1 M=1 mol dm−3) and 25 °C, r(SCN−)=100% at I=0.5 M and 25 °C, and r(N3−)=0% at I=0.55 M and 10 °C. These facts suggest that r depends considerably upon the bridging ability of X−; formation of binuclear intermediate may be favored by the symmetrical structure and the size of X−, which make the inner-sphere mechanism easier with the stronger coordinate bond and the weaker el...