Electron-Transfer in Outer-Sphere Reactions

Note that electron transfer in outer-sphere reactions between inorganic ions also shows a distance-dependence (see, e.g.. Equation (9.9), Section 9.1.2.1), attributable to changes in bond distances on forming the transient complex. 

Electron transfer in outer-sphere reactions of inorganic ions (Section 9.1) ... 

Since the overall reaction proceeds under conditions where no Co(CO)4 radicals from Co2(CO)g cleavage can be detected, splitting of the dinuclear species Co2(CO)gL, which is formed in a rapid preequilibrium (equation 4), is responsible for radical formation during the induction period. The important step for the formation of the observed products is then outer-sphere electron transfer see Outer-sphere Reaction) as depicted in equation (6). This requires the Co(CO)3L radical to act as a reducing agent towards Co2(CO)g. Since, from electrochemistry and pulse radiolysis of Co2(CO)g, it can... 

The field of modified electrodes spans a wide area of novel and promising research. The work dted in this article covers fundamental experimental aspects of electrochemistry such as the rate of electron transfer reactions and charge propagation within threedimensional arrays of redox centers and the distances over which electrons can be transferred in outer sphere redox reactions. Questions of polymer chemistry such as the study of permeability of membranes and the diffusion of ions and neutrals in solvent swollen polymers are accessible by new experimental techniques. There is hope of new solutions of macroscopic as well as microscopic electrochemical phenomena the selective and kinetically facile production of substances at square meters of modified electrodes and the detection of trace levels of substances in wastes or in biological material. Technical applications of electronic devices based on molecular chemistry, even those that mimic biological systems of impulse transmission appear feasible and the construction of organic polymer batteries and color displays is close to industrial use. 

The remarkable physical properties exhibited by the divalent macrobicyclic cage complex [Co(sep)]2+ (29) are unparalleled in Co chemistry.219 The complex, characterized structurally, is inert to ligand substitution in its optically pure form and resists racemization in stark contrast to its [Co(en)3]2+ parent. The encapsulating nature of the sep ligand ensures outer sphere electron transfer in all redox reactions. For example, unlike most divalent Co amines, the aerial oxidation of (29) does not involve a peroxo-bound intermediate. 

However, while remaining in the binding site, superoxide accepts its electron when the oxygen is more than 3 A from the copper ion (outer-sphere electron transfer) in the following reaction ... 

Romanian scientists compared one-electron transfer reactions from triphenylmethyl or 2-methyl benzoyl chloride to nitrobenzene in thermal (210°C) conditions and on ultrasonic stimulation at 50°C (lancu et al. 1992, Vinatoru et al. 1994, Chivu et al. 2006). In the first step, the chloride cation-radical and the nitrobenzene anion-radicals are formed. In the thermal and acoustic variants, the reactions lead to the same set of products with one important exception The thermal reaction results in the formation of HCl, whereas ultrasonic stimulation results in CI2 evolution. At present, it is difficult to elucidate the mechanisms behind these two reactions. As an important conclusion, the sonochemical process goes through the inner-sphere electron transfer. The outer-sphere electron transfer mechanism is operative in the thermally induced process. 

In outer-sphere reactions, electron transfer occurs without the utilization of a bridging ligand. The existence of such pathways is suggested for reactants which have no readily available coordination sites for forming a chemical bridge, as in equation (1), and where at least one of the reactants undergoes slow ligand substitution on the timescale for electron transfer, e.g. reaction (6). 

Redox reactions involving metal ions occur by two types of mechanisms inner-sphere and outer-sphere electron transfer. In inner-sphere mechanisms, the oxidant and reductant approach intimately and share a common primary hy-... 

The rate of formation of the precursor complex can be rate determining even if its formation does not involve ligand replacement. This occurs in outer-sphere reactions in which the electron transfer rate is rapid. The rate of formation of the precursor complex may then be diffusion controlled i.e., or, more generally, k = Kkj.,j/... 

In the case of reactions with Co(II), the difference in the rate of reduction can be explained by different mechanisms for the two reactions. The reduction of [Co(NH3)5(OH)] proceeds via inner-sphere mechanism Co(II) complexes are labile (see Fig. 21.1) whereas [Co(NH3)5(OH)] has OH ligand that can serve as a bridging ligand. The (Co(NH3)5(H20)] has no bridging ligands and the only mechanistic pathway for the electron-transfer is outer sphere. 

There are two types of electron transfer mechanisms for transition metal species, outer- and inner-sphere electron transfers. The outer-sphere electron transfer occurs when the outer coordination sphere (or solvent) of the metal centers is involved in transferring electrons. This type of transfer does not imply reorganization of the inner coordination sphere of either reactant. An example of this reaction is given in Equation (3.54) ... 

Reduction of O3 by [IrClg] , shown in Eq. (4), is first-order in both reactants and has a second-order rate constant of 1.7x 10 s" at 25.0 °C. ° The initial electron transfer is outer-sphere in nature and allows computation of a self-exchange rate of 4M s for the 03/Of couple. Comparisons of this rate constant with the results of other electron transfer reactions of O3 reveal that inner-sphere mechanisms are common. 

Equation (3,23) has an interesting application. The fact that it definitely does not hold for not very rapid electrode reactions points towards a change in the mechanism of electron transfer in an electrode reaction as compared to a homogeneous process. By way of an example, we can mention a transition from an outer-sphere to an inner-sphere mechanism, i.e. a mechanism substantially involving ligands. This change in the reaction path is caused by strong adsorption of the complex on the electrode[236]. 

A powerful application of outer-sphere electron transfer theory relates the ET rate between D and A to the rates of self exchange for the individual species. Self-exchange rates correspond to electron transfer in D/D (/cjj) and A/A (/c22)- These rates are related through the cross-relation to the D/A electron transfer reaction by the expression... 

The elementary electrochemical reactions differ by the degree of their complexity. The simplest class of reactions is represented by the outer-sphere electron transfer reactions. An example of this type is the electron transfer reactions of complex ions. The electron transfer here does not result in a change of the composition of the reactants. Even a change in the intramolecular structure (inner-sphere reorganization) may be neglected in many cases. The only result of the electron transfer is then the change in the outer-sphere solvation of the reactants. The microscopic mechanism of this type of reaction is very close to that for the outer-sphere electron transfer in the bulk solution. Therefore, the latter is worth considering first. 

Here, n denotes a number operator, a creation operator, c an annihilation operator, and 8 an energy. The first term with the label a describes the reactant, the second term describes the metal electrons, which are labeled by their quasi-momentum k, and the last term accounts for electron exchange between the reactant and the metal Vk is the corresponding matrix element. This part of the Hamiltonian is similar to that of the Anderson-Newns model [Anderson, 1961 Newns, 1969], but without spin. The neglect of spin is common in theories of outer sphere reactions, and is justified by the comparatively weak electronic interaction, which ensures that only one electron is transferred at a time. We shall consider spin when we treat catalytic reactions. 

In outer sphere electron transfer, the reactant is not adsorbed therefore, the interaction with the metal is not as strong as with the catal5d ic reactions discussed below. Hence, the details of the metal band structure are not important, and the couphng A(s) can be taken as constant. This is the so-called wide band approximation, because it corresponds to the interaction with a wide, structureless band on the metal. In this approximation, the function A(s) vanishes, and the reactant s density of states takes the form of a Lorentzian. The simation is illustrated in Fig. 2.3. 

Although somewhat more stable than its hexaammine relative, the air-sensitive [Co(en)3]2+ is still substitutionally labile and racemizes rapidly in solution. Chiral discrimination in its (racemic) solutions has been observed in outer sphere electron transfer reactions with optically active oxidants including [Coin(EDTA)], 209,210 [Cr(ox)3]3-,211,212 Co111 oxalate, malonate, and acetylacetonate (acac) complexes.213... 

The aim in solution studies on metalloprotein is to be able to say more about intermolecular electron transfer processes, first of all by studying outer-sphere reactions with simple inorganic complexes as redox partners. With the information (and experience) gained it is then possible to turn to protein-protein reactions, where each reactant has its own complexities... 

---