Electron transfer reactions 

Electron transfer is the very simple type of redox process, an instance being

A majority of reactions of this kind are extremely fast, but oxidation through some complexes (particularly of Co^III) is much slower.

The coordination sphere of one complex is substituted through a ligand bound to the other complex, that then acts like a bridge and might be transferred during the redox process, in an inner sphere process. For instance, isotopic labeling studies depict that the oxidation of aqueous Cr²⁺ with [Co^III(NH₃)₅Cl]²⁺ proceeds through a bridged species Cr-Cl-Co, the chlorine not exchange with free labeled Cl^- in solution but remain attached to the kinetically inert Cr^III result.

An inner sphere technique needs one of the reactants to be substitutionally labile, and a ligand which can act as a bridge. One test is to evaluate the rates of reaction with the ligands azide N₃^-and (N bonded) thiocynanate NCS^-; azide is usually fine at bridging and so provides faster rates if the inner sphere route is operating.

The outer sphere technique includes no disruption of the coordination of either complex, and is all the time available like a route to electron transfer except the inner sphere rate is faster. The Marcus theory depicts that the rate of outer sphere transfer relies on:

(i) The orbital interaction among the two metal centers included, a issue that decreases approximately exponentially with the distance among them;

(ii) The alteration in metal-ligand distances resultant from electron transfer, the influence that gives most of the activation energy for the reaction;

(iii) An improvement term, that depends on the variation of redox potentials of the two couples involved.

Reactions of complexes consisting of unsaturated ligands like bipyridyl are usually fast because the π system makes easy transfer, and since the change in geometry is small (like over the ligand significant charge is distributed). Alternatively, oxidation through [Co(NH₃)₆]³⁺ is frequently very slow. The orbital interaction term is small since the reaction

is 'spin forbidden', the ground state of Co varying from low-spin d⁶ along with no unpaired electrons to high-spin Co²⁺ d⁷ along with three. The activation energy is also large due to the number of eg electrons increases by two that provides a significant change of LFSE and so causes a large increase in the metal-ligand distances. The inner sphere route is not available as NH₃ does not usually act as a bridging ligand.