Successor complex outer-sphere electron transfer

Figure 93 Three-step mechanism for outer-sphere electron-transfer reactions. Line 1. Chemical species and processes. Line 2. R are the reactants I is the precursor intermediate complex I is the successor intermediate complex P are the products j is the activated complex. Line 3. a is the association to I et is the electron-transfer step d is the dissociation of I to products b, c is the ligand and solvent reorganisation. Line 4. Free-energy changes.

The homogeneous outer sphere electron transfer reactions in solution occur at a rate that is noticeably Icj er than the diffusion rate. This peculiar behaviour has been explained through a three-step mechanism formation of a precursor complex from the separated reactants, actual electron transfer within this complex to form a successor complex and dissociation of the latter complex into separated products. The reaction rate is usually controlled by the electron transfer step, this step being governed by the Franck-Condon principle. This principle is embodied in classical electron transfer theories using an activated-complex formalism in which the electron transfer occurs at the intersection of two potential energy surfaces, one for the reactants and the other for the products. This implies that the second step necessarily involves the reorganization of the solvent before and after the electron transfer itself is produced. So, it is obvious that solvent must play an essential role in the rate of electron transfer reactions in solution. 

Similarly, inner-sphere and outer-sphere mechanisms can be postulated for the reductive dissolution of metal oxide surface sites, as shown in Figure 2. Precursor complex formation, electron transfer, and breakdown of the successor complex can still be distinguished. The surface chemical reaction is unique, however, in that participating metal centers are bound within an oxide/hydroxide... 

The rate-controlling step in reductive dissolution of oxides is surface chemical reaction control. The dissolution process involves a series of ligand-substitution and electron-transfer reactions. Two general mechanisms for electron transfer between metal ion complexes and organic compounds have been proposed (Stone, 1986) inner-sphere and outer-sphere. Both mechanisms involve the formation of a precursor complex, electron transfer with the complex, and subsequent breakdown of the successor complex (Stone, 1986). In the inner-sphere mechanism, the reductant... 

Reaction between the one electron reduced POM anions and O2 were then summarized. These studies demonstrated that at mildly acidic to neutral pH values in water, electron transfer from Keggin heteropolytungstates to O2 occurs by an outer-sphere mechanism. In these cases, the effect of Keggin-ion charge on rate constants for the reduction of O2 was shown to be significant, and was attributed to anion-anion repulsion within the successor-complex ion pairs formed between the negatively charged POM anions and 02 . This was followed by the discovery of a concerted proton-electron (CPET) pathway for electron transfer to O2 at lower pH values (< 1). 

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