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<i>vs.</i>Inner-sphere Electron Transfer Mechanisms in the Presence of X<sup>−</sup>(X<sup>−</sup>=Cl<sup>−</sup>, SCN<sup>−</sup>, and N<sub>3</sub><sup>−</sup>) in Aqueous Solutions, as Judged from the Kinetics of the Dissociation Reactions of FeX<sup>2+</sup>
作者:Goro Wada、Tomiko Inatani、Sachiko Ichimura
DOI:10.1246/bcsj.55.3441
日期:1982.11
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 electric repulsion between the two reactants beyond the bridging ligand.
Fe²⁺与FeX²⁺(X⁻=Cl⁻、SCN⁻或N₃⁻)之间的电子转移反应可能通过内球或外球机制发生,或两者同时进行。外球机制的过程在表面上与Fe²⁺催化的FeX²⁺解离过程等效。当电子转移反应的总体速率常数和Fe²⁺辅助解离反应的速率常数分别用kE和ko表示时,外球机制在总反应中的比例r可以用r=2ko/kE表示。因此,r值的测定结果显示,r(Cl⁻)=44%(I=1.5 M,1 M=1 mol dm⁻³,25°C),r(SCN⁻)=100%(I=0.5 M,25°C),以及r(N₃⁻)=0%(I=0.55 M,10°C)。这些事实表明,r值在很大程度上依赖于X⁻的桥接能力;X⁻的对称结构和大小可能有利于形成双核中间体,从而使内球机制更容易发生,两个反应物之间的坐标键更强、桥接配体以外的电排斥力更弱。