Inner-sphere pathway, definition

Figure 1 depicts schematically the interaction of solvated ions with the electrode surface both for outer sphere (a) and inner sphere pathways (b). Notice, however, that some ambiguity is found with respect to the previous definitions for case (c) in which the coordination sphere of the reacting ion penetrates the layer of solvent molecules adjacent to the electrode, but the ligand is the same solvent molecule and therefore cannot be distinguished from the inner layer of solvent molecules. This may be considered an outer sphere pathway unless the solvent ligand adjacent to the electrode is not present in the product of reaction. 

Outer-sphere pathways are normally expected to be of the weak-overlap type since reactant-electrode bond formation is, by definition, absent. Nevertheless, substantial energy differences between 0 and P, and R and S may occur (so-called "double-layer effects, Sect. 3.5.2), at least as a result of electrostatic reactant-surface interactions. Inner-sphere pathways may be either of the weak- or strong-overlap type, depending on the strength and specificity of the reactant-surface binding (Sect. 3.5.1). 

Classical definitions also become uncertain with nonmetallic reagents. Because such species commonly are neutral or anionic, solvation is less striking and almost always labile. When does Br utilize an inner-sphere pathway A consistent definition is possible if the transition state for the oxidation of Fe by Br is ... 

The rate constants for such outer-sphere reactions can therefore differ markedly from those corresponding to true weak-overlap pathways, even after correction for electrostatic double-layer effects. This can cause some difficulties with the operational definition of inner-sphere electrocatalysis considered above, whereby outer-sphere reactions are regarded as "non-catalytic processes. In addition, there is evidence that inner- rather than outer-sphere pathways can provide the normally preferred pathways at metal-aqueous interfaces for reactants containing hydrophobic functional groups [116]. 

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