Redox reactions outer-sphere mechanism

Other applications of Marcus theory include calculation of energy barriers in micelle reactions, with total micellar charges estimated via work terms, " assignment of outer-sphere mechanisms in reactions of organic radicals, and calculation of the unknown redox potential of one reagent, either by curve fitting, or by a linear extrapolation of rates to the diffusion-controlled limit. ... 

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). 

TABLE 8-1. Preference for the conduction band mechanism (CB) and the valence band mechanism (VB) in outer sphere electron transfer reactions of hydrated redox particles at semiconductor electrodes (SC) Eo = standard redox potential referred to NHE c, = band gap of semiconductors. [From Memming, 1983.]... 

Only in a limited number of instances will the value of k and its associated parameters be useful in diagnosing mechanism. When the redox rate is faster than substitution within either reactant, we can be fairly certain that an outer-sphere mechanism holds. This is the case with Fe + and RuCP+ oxidation of V(II) and with rapid electron transfer between inert partners. On the other hand, when the activation parameters for substitution and redox reactions of one of the reactants are similar, an inner-sphere redox reaction, controlled by replacement, is highly likely. This appears to be the case with the oxidation by a number of Co(III) complexes of V(II), confirmed in some instanees by the appearance of the requisite V(III) complex, e.g. 

Another example is the reaction of indoles with nitrosoarenes in the presence of acids. The redox potentials of the reactants cannot justify an outer-sphere ET process, thus the formation of the phenylaminoxyl detected for the reaction carried out in the ESR cavity, could be more likely justified by an inner-sphere ET mechanism95. In fact the reaction of quinoline N-oxide with primary alkyl Grignards for which an outer-sphere mechanism was earlier proposed, takes place through classical nucleophilic addition96. 

There has been some exploration of the mechanism of reduction of d transition metal complexes by M2+(aq) (M = Eu, Yb, Sm). Both inner- and outer-sphere mechanisms are believed to operate. Thus the ready reduction of [Co(en)3]3+ by Eu2+(aq) is necessarily outer-sphere. 2 However, the strong rate dependence on the nature of X when [Co(NH3)5X]2+ or [Cr(H20)5X]2+ (X = F, Cl, Br or I) are reduced by Eu2+(aq) possibly suggests an inner-sphere mechanism.653 The more vigorous reducing agent Yb2+ reacts with [Co(NH3)6]3+ and [Co(en)3]3+ by an outer-sphere route but with [Cr(H20)5X]2+ (X = halide) by the inner-sphere mechanism.654 Outer-sphere redox reactions are catalyzed by electron-transfer catalysts such as derivatives of isonicotinic acid, one of the most efficient of which is iV-phenyl-methylisonicotinate, as the free radical intermediate does not suffer attenuation through disproportionation. Using this catalyst, the outer-sphere reaction between Eu2+(aq) and [Co(py)(NH3)5]3+ proceeds as in reactions (18) and (19). Values found were ki = 5.8 x KFM-1 s 1 and k kx = 16.655... 

As discussed in Vol. 2, Chap. 4, experimental studies, mainly pioneered by Taube [11], revealed two different reaction pathways for redox reactions in solution (i) outer sphere mechanism characterized by weak interaction of the reactive species, with the inner coordination sphere remaining intact during the electron transfer, and reactions occurring through a common ligand shared by the metallic centers thus proceeding by an inner sphere mechanism. 

Tris(l,2-bis(dimethylphosphino)ethane)rhenium(I), [Re(DMPE)3]+ is a simple, symmetrical cation which contains three identical bidentate phosphine ligands. This complex provides a Re(II/I) redox couple with properties that are very convenient for the study of outer-sphere electron transfer reactions.1 Specifically, this couple is stable in both alkaline and acidic media and it exhibits a reversible, one-equivalent redox potential in an accessible region [ °,(II/I) = 565 mV vs. NHE]. Moreover, this complex has been used to obtain information about the biological mechanism of action of 186Re and l88Re radiopharmaceuticals.2,3... 

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... 

A number of situations may be visualized. Electron transfer may take place between a pair of redox proteins in solution. Certain reactions in the cytoplasm of the red blood cell fall into this category, such as that between hemoglobin and cytochrome b reductase. These reactions will probably occur by an outer-sphere mechanism, as was described earlier for model reactions between isolated electron-transfer proteins and also between these proteins and simple complexes. Interaction between such proteins probably utilizes specific charged areas on their surfaces. The possibility of inner-sphere reactions may have to be considered in a few cases. 

A large number of radical reactions proceed by redox mechanisms. These all require electron transfer (ET), often termed single electron transfer (SET), between two species and electrochemical methods are very useful to determine details of the reactions (see Chapter 6). We shall consider two examples here - reduction with samarium di-iodide (Sml2) and SRN1 (substitution, radical-nucleophilic, unimolecular) reactions. The SET steps can proceed by inner-sphere or outer-sphere mechanisms as defined in Marcus theory [19,20]. 

Typically, however, the redox reactions are induced by the CT states. The reactions can proceed via an inner- or outer-sphere mechanism (see Figure 6.6), irrespective of the initial mode the redox processes are usually accompanied by reactions with exterior molecules, induced by the decreased coordination ability of the centre, or ligand, or both. This manifests in successive reactions with solvents or other accessible ligands present in the medium [51, 52] (Figure 6.7). 

It is generally believed that the oxidation of thiourea and related compounds by aqua-metal ions involves an inner-sphere electron-transfer process, whereas an outer-sphere mechanism is more commonly associated with substitution-inert complexes. The stoichiometry of redox reactions with one-electron oxidizing agents is different for acid and alkaline media. The oxidation of both thiourea and thioacetamide by [Mo(CN)g] in the range 0.02 < [HCIO4] < 0.08 M proceeds in a 1 1 ratio, yielding the disulfide as a product (108) ... 

Redox processes between metal complexes are divided into outer-sphere processes and inner-sphere processes that involve a ligand common to both coordination spheres. The distinction is fundamentally between reactions in which electron transfer takes place from one primary bond system to another (outer-sphere mechanism) and those in which electron transfer takes place within a primary bond system (inner-sphere mechanism) (Taube, 1970). 

Figure 2 shows this for oxidation of various substituted phenanthroline Fe(II) complexes by Ce(IV). An average rate constant of 2 x 103 M 1 s -1 was found for the phenanthroline-Fe(II)-FE(III) exchanges by this approach (Dulz and Sutin, 1963) this compares with a value of 4 M i s-1 for the free ions. Many studies have verified the Marcus relationship for metal ion redox reactions, and large deviations are assumed to indicate that the reaction occurs by an inner-sphere mechanism. (Note An outer-sphere mechanism can be inferred if the redox reaction is faster than the rates of ligand exchange for the metal ions.)... 

If there is a strong electronic coupling between P and R in the transition state, one commonly speaks of an inner-sphere mechanism, and, conversely, if the interaction is weak, one uses the term outer-sphere mechanism. There are various theoretical approaches for quantifying the rates of redox reactions, including the so-called Marcus theory. For a description of these approaches, we refer to the literature (e.g., Eberson, 1987). For our discussion here, we content ourselves with trying to identify the factors that determine the rate at which a given organic pollutant is reduced or oxidized in the environment. 

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. 

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