Outer-sphere mechanism for electron-transfer

Inner-sphere and Outer-sphere Mechanisms for Electron Transfer... 

A bridging ligand reduction model vs. the outer-sphere mechanism for electron transfer has been tested using rate constants from Cr(II) systems. A correlation between the rate constant and the gas-phase electron affinity of the bridging group implies an inner-sphere mechanism. If such a correlation is absent an outer-sphere mechanism is assumed. ... 

Outer-Sphere Electron Transfer The minimal interpenetration of the coordination spheres of the reactants is inherent in any mechanistic formulation of the outer-sphere process for electron transfer. As such, steric effects provide a basic experimental criterion to establish this mechanism. Therefore we wish to employ the series of structurally related donors possessing the finely graded steric and polar properties described in the foregoing section for the study of both homogeneous and heterogeneous processes for electron transfer. 

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,... 

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

Cyano complexes have been involved in many kinetic studies.1 The fast electron transfer reactions between [Fe(CN)6]3- and [Fe(CN)6]4-, and between [Mo(CN)8]3- and [Mo(CN)g]4-, for example, were important in establishing the outer-sphere mechanism for redox reactions. The kinetics of... 

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 reaction mechanisms of Fe-SOD and Mn-SOD are not yet as well characterized as those of CuZn-SOD, but the ligands of the metals have been confirmed to be three histidyl immidazoles and one aspartyl carboxylate. The Fe- and Mn-SODs undergo alternate reduction and reoxidation during the catalytic cycle, similar to CuZn-SOD, at a rate constant of 2X109 M s l. The electron transfer to O2 from the metals proceeds through an outer sphere mechanism for Fe- and Mn-SODs, while it proceeds via an inner sphere mechanism for CuZn-SOD. Fe- and Mn-SODs exhibit substrate saturation77 as does CuZn-SOD. 

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. 

There are two well established general mechanisms for electron-transfer processes. In the first, called the outer-sphere mechanism, each complex retains its own full coordination shell, and the electron must pass through... 

Studies of redox reactions involving transition metals in solution have established that there are basically two types of mechanism for electron transfer, outer sphere and inner sphere mechanisms. In addition to this distinction, redox reactions are divided into self exchange reactions between two different oxidation states of the same metal ion and redox reactions between complexes with different metal centres. In the following sections the mechanisms of these reactions will be discussed in conjunction with a discussion of the factors which influence the rates of redox reactions. 

The kinetics if the anaerobic reduction of stellacyanin, plastocyanin, azurin, and laccase by [Fe(edta)] have been reported. Simple second-order behaviour was observed and the following rate constants, with their associated and values, were measured (at 25 °C and pH 7) 4.3x10 , 8.2x10 , 1.3x10 , and 2.6X 10 1 mol" s 3, 2, 2, and 13 kcal mol" and -21, -29, -37, and -5 cal K mol", respectively. The authors favour an outer-sphere mechanism for azurin, plastocyanin, and stellacyanin but conclude that laccase employs a pathway which requires specific protein activation (of ca. lOkcalmol" in A/f ) to accept the reductant. The kinetics of the reduction of laccase by [Fe(CN)6] are complicated, as they are for the autoxidation of reduced laccase. The results - for electron transfer between azurin and cytochrome c have already been mentioned. 

An outer-sphere mechanism for the oxidation of L-ascorbic acid (HA) with acidic [PtCls] " involves the transfer of two electrons from HA to the Pt(IV) centre with release of two Cr ions and simultaneous formation of square planner Pt(II) halide and dehydrated ascorbic acid. ... 

In view of the above arguments and the kinetic interpretations, the more suitable oxidation reaction mechanism, which agrees with the experimental kinetic results, involves a fast protonation of the alcoholic groups to form the more reactive alkoxnium ions prior to the rate-determining steps. Such protonation processes are followed by the attack of Cr (VI) ion on the centers of reactive alkoxnium ions forming its corresponding ester-like species as discussed before. The decomposition of the formed coordination biopolymer intermediates (ester-like species) takes place by two possible reaction mechanisms for electron transfer. The first one corresponds to an outer-sphere mechanism in which the transfer... 

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. 

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). 

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,... 

---