Electron-transfer in outer-sphere reactions of metal ions

Electron-transfer in outer-sphere reactions of metal ions 

Among oxidation-reduction reactions in solution, the formally simplest type is that in which an electron is transferred between two ions of variable valency, without change in the number or the nature of the ligands attached to the ions. These reactions have been intensively studied, mainly in aqueous solution. Some examples of such electron-transfer reactions are  

The last of these reactions, in which only a single element undergoes chemical change, is of the type variously known as a self-reaction or self-exchange or synunelrical electron transfer. The reactions are reversible and many of them can be made to occur in an electrochemical cell comparisons can then be made between the kinetics at an electrode and those in homogeneous solution. Many of these reactions can be studied only by fast-reaction techniques. Stopped-flow, quenching, temperature-jump, flash, pulse radiolysis, and nmr techniques have all been used. 

Some electron-transfer reactions have been shown to have mechanisms that involve a bridged intermediate, for example [1]  

Others do not, as is clearly shown when the reaction is faster than substitution into the coordination shell of either ion for instance, reaction (9.2) is very rapid although both ions undergo substitution only slowly. Reactions in which no bridge can be suspected are called outer-sphere electron transfers. They are in some ways uniquely simple, in that no covalent bonds are made or broken, and very little mass is transferred. The fact that a charge is carried by the electron, however, has large and complex effects [2]. 

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