Mechanisms outer sphere

When both reactants in a redox reaction are kinetically inert, electron transfer must take place by a tunnelling or outer-sphere mechanism. For a reaction such as 25.46, AG° 0, but activation energy is needed to overcome electrostatic repulsion between ions of like charge, to stretch or shorten bonds so that they are equivalent in the transition state (see below), and to alter the solvent sphere around each complex. 

In a self-exchange reaction, the left- and right-hand sides of the equation are identical only electron transfer, and no net chemical reaction, takes place. 

Clearly, the reactants must approach closely for the electron to migrate from reductant to oxidant. However, there is an important restriction imposed by the Franck-Condon principle, during electron transfer, the nuclei are essentially 

Second order rate constants, k, for some outer-sphere redox reactions at 298 K in aqueous solution. 

By the Franck-Condon principle, a molecular electronic transition is much faster than a molecular vibration. 

The accepted method of testing for an outer-sphere mechanism is to apply Marcus-Hmh theory which relates kinetic and thermodynamic data for two self-exchange reactions with data for the cross-reaction between the selfexchange partners, e.g. reactions 26.56-26.58. 

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The superb elegance of this demonstration lies in the choice of reactants which permits no alternative mechani.sm. Cr" (d ) and Co" (d ) species are known to be substitutionally labile whereas Cr" (d ) and Co " (low-spin d ) are substitutionally inert, Only if electron transfer is preceded by the formation of a bridged internrediate can the inert cobalt reactant be persuaded to release a Cl ligand and so allow the quantitative formation of the (then inert) chromium product. Corroboration that electron transfer does not occur by an outer-sphere mechanism followed by los.s of CP from the chromium is provided by the fact that, if Cl is added to the solution, none of it finds its way into the chromium product. 

On the basis of these results it seems to the present author that inner and outer complexes can reasonably be assumed for the electron transfer to the diazonium ion, but that an outer-sphere mechanism is more likely for metal complexes with a completely saturated coordination sphere of relatively high stability, such as Fe(CN) (Bagal et al., 1974) or ferrocene (Doyle et al., 1987 a). Romming and Waerstad (1965) isolated the complex obtained from a Sandmeyer reaction of benzenediazonium ions and [Cu B ]- ions. The X-ray structural data for this complex also indicate an outer-sphere complex. 

The Marcus treatment applies to both inorganic and organic reactions, and has been particularly useful for ET reactions between metal complexes that adopt the outer-sphere mechanism. Because the coordination spheres of both participants remain intact in the transition state and products, the assumptions of the model are most often satisfied. To illustrate the treatment we shall consider a family of reactions involving partners with known EE rate constants. 

In the same way that we considered two limiting extremes for ligand substitution reactions, so may we distinguish two types of reaction pathway for electron transfer (or redox) reactions, as first put forth by Taube. For redox reactions, the distinction between the two mechanisms is more clearly defined, there being no continuum of reactions which follow pathways intermediate between the extremes. In one pathway, there is no covalently linked intermediate and the electron just hops from one center to the next. This is described as the outer-sphere mechanism (Fig. 9-4). 

Figure 9-4. The outer-sphere mechanism for an electron transfer reaction between two complexes. No covalently-linked intermediate is involved in the reaction.

Endicott and Taube consider that there is cause for doubt over the generally-held views that Cr(bipy)3 is oxidised by an outer-sphere mechanism"". They suggest that, since the complex is very labile to substitution, coordination sites... 

The aquation of [IrCl6]2- to [Er( E120)C1S] and Ir(H20)2Cl4 has been found to activate the complex toward the oxidation of insulin in acidic solutions, with measured rate constant of 25,900 and 8,400 Lmol-1 s 1, respectively.50 The oxidation reaction proceeds via an outer-sphere mechanism. 

Electron transfer between metal ions contained in complexes can occur in two different ways, depending on the nature of the metal complexes that are present. If the complexes are inert, electron transfer occurring faster than the substitution processes must occur without breaking the bond between the metal and ligand. Such electron transfers are said to take place by an outer sphere mechanism. Thus, each metal ion remains attached to its original ligands and the electron is transferred through the coordination spheres of the metal ions. 

Electron Transfer Far From Equilibrium. We have shown how the Marcus Theory of electron transfer provides a quantitative means of analysis of outer-sphere mechanisms in both homogeneous and heterogeneous systems. It is particularly useful for predicting electron transfer rates near the equilibrium potential,... 

Cytochrome c is responsible for accepting an electron from cytochrome Ci and transferring it to cytochrome c oxidase. The electron transfer reaction may occur via the exposed portion of the ring or by tunnelling through the protein (and involving an outer-sphere mechanism). The details of this process have not been fully elucidated and have remained the focus of much research. 

In aqueous solutions these reactions seem to proceed via an outer-sphere mechanism on most metals. Typically such reactions involve metal ions surrounded by inert ligands, which prevent adsorption. Note that the last example reacts via an outer-sphere pathway only if trace impurities of halide ions are carefully removed from the solution otherwise it is catalyzed by these ions. 

The transfer coefficient a has a dual role (1) It determines the dependence of the current on the electrode potential. (2) It gives the variation of the Gibbs energy of activation with potential, and hence affects the temperature dependence of the current. If an experimental value for a is obtained from current-potential curves, its value should be independent of temperature. A small temperature dependence may arise from quantum effects (not treated here), but a strong dependence is not compatible with an outer-sphere mechanism. 

This gap in our knowledge is now closed, as the first paper on the uncatalyzed outer-sphere oxidation of an aliphatic thiol was recently published (12).This work selected thioglycolic acid (TGA, mercaptoacetic acid, HSCH2CO2H) as a representative thiol because of its high water solubility, low vapor pressure, and simple structure. The oxidant was [IrCle]2-, a well-characterized one-electron oxidant that frequently reacts through an outer-sphere mechanism. As is typical of such... 

Iron(III)-catalyzed autoxidation of ascorbic acid has received considerably less attention than the comparable reactions with copper species. Anaerobic studies confirmed that Fe(III) can easily oxidize ascorbic acid to dehydroascorbic acid. Xu and Jordan reported two-stage kinetics for this system in the presence of an excess of the metal ion, and suggested the fast formation of iron(III) ascorbate complexes which undergo reversible electron transfer steps (21). However, Bansch and coworkers did not find spectral evidence for the formation of ascorbate complexes in excess ascorbic acid (22). On the basis of a combined pH, temperature and pressure dependence study these authors confirmed that the oxidation by Fe(H20)g+ proceeds via an outer-sphere mechanism, while the reaction with Fe(H20)50H2+ is substitution-controlled and follows an inner-sphere electron transfer path. To some extent, these results may contradict with the model proposed by Taqui Khan and Martell (6), because the oxidation by the metal ion may take place before the ternary oxygen complex is actually formed in Eq. (17). 

Most of the kinetic models predict that the sulfite ion radical is easily oxidized by 02 and/or the oxidized form of the catalyst, but this species was rarely considered as a potential oxidant. In a recent pulse radiolysis study, the oxidation of Ni(II and I) and Cu(II and I) macrocyclic complexes by SO was studied under anaerobic conditions (117). In the reactions with Ni(I) and Cu(I) complexes intermediates could not be detected, and the electron transfer was interpreted in terms of a simple outer-sphere mechanism. In contrast, time resolved spectra confirmed the formation of intermediates with a ligand-radical nature in the reactions of the M(II) ions. The formation of a product with a sulfonated macrocycle and another with an additional double bond in the macrocycle were isolated in the reaction with [NiCR]2+. These results may require the refinement of the kinetic model proposed by Lepentsiotis for the [NiCR]2+ SO/ 02 system (116). 

Figure 13.18 S-adenosyl methionine (SAM), a source of 5 -deoxyadenosyl radicals. SAM binds to the subsite iron (in blue) of the reduced [4Fe-4S] cluster via its a-aminocarboxylate group. The 5 -deoxyadenosine radical is formed by electron transfer which occurs either (a) by outer-sphere mechanism or (b) by p-sulfide alkylation followed by homolytic cleavage of the 5 -S-CH2Ado bond. In both cases, methionine is released. (From Fontecave et al., 2004. Copyright 2004, with permission from Elsevier.)...

Electron transfer reactions may follow two types of mechanism (i) outer sphere mechanism and (ii) inner sphere mechanism. 

In principle, outer sphere mechanism involves electron transfer from reductant to oxidant when there is no change in the number or nature of the groups (coordination shells or spheres) attached to each of them. For example,... 

In outer sphere mechanism, one reactant becomes involved in the outer or second coordination sphere of the other reactant and an electron flows from the reductant to oxidant. It is also possible that the electron is transferred first to the solvent and then from the solvent to an ion. 

The outer sphere mechanism may take place in all redox active systems while inner sphere mechanism requires substitutionally labile reactants and products. 

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... 

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