Successor complex inner-sphere electron transfer

The Marcus theory model is derived for unimolecular electron transfer. It is applied to bimolecular reactions by assuming that the reactants weakly associate in a precursor complex within which ET occurs to give the successor complex. The cross relation analyses above have implicitly adopted this same model, but HAT precursor complexes are quite different then ET ones. This is because proton transfer occurs only over very short distances, so HAT precursor complexes have distinct conformations, rather than the weakly interacting encounter complexes of ET. In this way, HAT resembles proton transfer and inner-sphere electron transfer. Including the equilibria for precursor and successor complex formation expands equation (1.1) into equation (1.20). 

Inner-sphere. Here, the two reactants first form a bridged complex (precursor)- intramolecular electron transfer then yields the successor which in turn dissociates to give the products. The first demonstration of this was provided by H. Taube. He examined the oxidation of ICrfHoOijj by lCoCl(NHr)< and postulated that it occurs as follows ... 

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... 

For an inner-sphere reaction there are necessarily more steps since both association and substitution must precede electron transfer. Intermediates like (H20)5CruClCoUI(NH3)54+ and (H20)5CrinClCoII(NH3)54 shown in Scheme 2 are often referred to as the precursor and successor complexes since they precede or follow the electron transfer step. 

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... 

The first step involves the formation of the precursor complex, where the reactants maintain their identity. In the second step there is, as we will see later, reorganization of the inner coordination shells as well as of the solvation spheres of the reactants so as to obtain a nuclear configuration appropriate to the activated complex through which the precursor complex is transformed into the successor complex. The electron transfer usually occurs during the latter stages of this reorganization process. The activated complex deactivates to form the successor complex if electron transfer has occurred or to reform the precursor complex if electron transfer has not occurred. The electron distribution in the successor complex corresponds to that of the products, so that the third step is simply the dissociation of the successor complex to form the separated products. 

Under favorable conditions, then, the three possible transition states for net inner sphere can differ in composition for the usual interchange substitution mechanism there are (1) precursor formation, [ALXB] (2) electron transfer, [AXB] and (3) successor breakdown, [AXL B]. However, L and L usually are solvent, and the number of solvent molecules in an activated complex cannot be determined kinetically. Therefore, under this normal circumstance all three possible transition states have the same composition and cannot be distinguished by direct kinetic measurements only indirect arguments can be used to determine which of the three possible transition states is operative. 

In principle, any of the steps in Scheme 2 can be rate limiting from the diffusion controlled formation of the association complex to the substitutional breakup of the successor complex. Because of the multiplicity of steps, a detailed interpretation of an experimentally observed rate constant can be extremely difficult. However, in some cases it has been possible to obtain direct or indirect information about the electron transfer step in an inner-sphere reaction using chemically prepared, ligand-bridged dimeric complexes. For example, reduction of the Co -Ru precursor to the product shown in equation (7) by Ru(NH3)6 + or Eu + occurs selectively at the Ru " site. The initial reduction to give Ru is followed by intramolecular electron transfer from Ru to Co which is irreversible since the Co site is rapidly lost by aquation. ... 

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