Electron-transfer processes inner-sphere mechanism

In terms of the development of an understanding of the reactivity patterns of inorganic complexes, the two metals which have been pivotal are platinum and cobalt. This importance is to a large part a consequence of each metal having available one or more oxidation states which are kinetically inert. Platinum is a particularly useful element of this pair because it has two kinetically inert sets of complexes (divalent and tetravalent) in addition to the complexes of platinum(O), which is a kinetically labile center. The complexes of divalent and tetravalent platinum show significant differences. Divalent platinum forms four-coordinate planar complexes which have a coordinately unsaturated 16-electron d8 platinum center, whereas tetravalent platinum is an 18-electron d6 center which is coordinately saturated in its usual hexacoordination. In terms of mechanistic interpretation one must therefore consider both associative and dissociative substitution pathways, in addition to mechanisms involving electron transfer or inner-sphere atom transfer redox processes. A number of books and articles have been written about replacement reactions in platinum complexes, and a number of these are summarized in Table 13. 

The second mechanism involves the formation of a covalent bridge through which the electron is passed in the electron transfer process. This is known as the inner-sphere mechanism (Fig. 9-5). 

A more interesting situation is found when the homogeneous redox reaction is combined with a chemical reaction between the electrocatalyst and the substrate. In this case, the catalytic process is called chemical catalysis. 3 This mechanism is depicted in Scheme 2 for reduction. The coupling of the electron transfer and the chemical reaction takes place via an inner-sphere mechanism and involves the formation of a catalyst-substrate [MC-S] complex. Here the selectivity of the mechanism is determined by the chemical step. Metal complexes are ideal candidates... 

Traditionally, electron transfer processes in solution and at surfaces have been classified into outer-sphere and inner-sphere mechanisms (1). However, the experimental basis for the quantitative distinction between these mechanisms is not completely clear, especially when electron transfer is not accompanied by either atom or ligand transfer (i.e., the bridged activated complex). We wish to describe how the advantage of using organometals and alkyl radicals as electron donors accrues from the wide structural variations in their donor abilities and steric properties which can be achieved as a result of branching the alkyl moiety at either the a- or g-carbon centers. 

The prospects for electron transfer mechanisms clearly extend beyond inorganic chemistry into the broad regions of organometal-lic and organic systems. Pushed to these limits, adequate quantitative criteria will be needed to delineate outer-sphere from inner-sphere mechanisms. However, the extent to which theoretical studies will provide more concrete guidelines of predictive value will determine whether electron transfer processes will form the basis of reaction mechanisms into the next century. 

It has in general been the objective of many mechanistic studies dealing with inorganic electron-transfer reactions to distinguish between outer- and inner-sphere mechanisms. Along these lines high-pressure kinetic methods and the construction of reaction volume profiles have also been employed to contribute toward a better understanding of the intimate mechanisms involved in such processes. The differentiation between outer- and inner-sphere mechanisms depends... 

To summarize, in this article we have discussed some aspects of a semiclassical electron-transfer model (13) in which quantum-mechanical effects associated with the inner-sphere are allowed for through a nuclear tunneling factor, and electronic factors are incorporated through an electronic transmission coefficient or adiabaticity factor. We focussed on the various time scales that characterize the electron transfer process and we presented one example to indicate how considerations of the time scales can be used in understanding nonequilibrium phenomena. 

One final point should be noted. Theoretical discussions of electron transfer processes have focused almost entirely on outer-sphere processes. When we have an inner-sphere mechanism, or sufficient electronic interaction in a dynamically trapped mixed-valence complex to produce a large separation between upper and lower potential surfaces, the usual weak-interaction approach has to be abandoned. Thus a detailed knowledge of a potential surface which is not describable as an intersection surface of perturbed harmonic surfaces, for example, is required. For this purpose, detailed calculations will be required. The theory of these processes will be linked more... 

In this picture, the electron transfer processes mediated by metallic electrodes (redox reactions in a heterogeneous phase) can also be classified to proceed according to outer-sphere or inner-sphere mechanisms (obviously, considering the electrode surface as a reagent). 

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. 

The Marcus treatment uses a classical statistical mechanical approach to calculate the activation energy required to surmount the barrier. It assumes a weakly adiabatic electron transfer process and non-equilibrium dielectric polarization of the solvent (continuum) as the source of activation. This model also considers the vibrational contributions of the inner solvation sphere. The Hush treatment considers ion-dipole and ligand field concepts in the treatment of inner coordination sphere contributions to the energy of activation [55, 56]. 

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... 

In recent years, there has been a great deal of interest in the mechanisms of electron transfer processes.52-60 It is now recognized that oxidation-reduction reactions involving metal ions and their complexes are mainly of two types inner-sphere (ligand transfer) and outer-sphere (electron transfer) reactions. Prototypes of these two processes are represented by the following reactions. 

Further work by Flowers examined the role of solvent polarity in the electron transfer process.30 Inner-sphere electron transfer kinetics show a weak dependence on solvent polarity due to the considerable orbital overlap of the donor-acceptor pair in the transition state. In an outer-sphere process, changes in solvent polarity alter the energetics of electron transfer. The addition of excess HMPA, beyond that required to saturate Sml2, resulted in a linear correlation to the rate of reduction for alkyl iodides, whereas no impact was observed on the rate of ketone reduction.30 Thus the experiments showed a striking difference in the electron transfer mechanism for the substrate classes, which is consistent with the operation of an outer-sphere-type process for the reduction of alkyl iodides and an inner-sphere-type mechanism for the reduction of ketones.30 These findings are consistent with the observations of Daasbjerg and Skrydstrup.28,29... 

If, whatever the interest of conceiving electron-pair transfer reactions such as Sn2 substitution as an inner sphere electron-transfer process, single electron transfer is intended to qualify reactions in which the rate-determining step is an outer sphere, non-dissociative or dissociative electron-transfer step preceding the bond-formation step, then the answer is no. There are a number of cases where true SN2 mechanisms (in which the bond-breaking and bond-formation steps are concerted) do occur, even with nucleophiles that are members of reversible one-electron reversible redox couples. In terms of activation energy, the SN2 mechanism merges with the outer sphere, dissociative electron-transfer mechanism when the bonded interactions in the transition state vanish. Steric constraints at the electro-... 

Solutions of indium (I) can be prepared by treatment of indium amalgam with silver triflate in dry acetonitrile in the absence of oxygen, and then diluted with water to give the low-concentration aqueous solution, which plays a sizable role in the study of the details of intermolecular electron transfer processes in solution. Aqueous In(I) solution has been used to examine the behavior of this hypovalent center in inorganic redox transformations. Reactions with complexes of the type [(NH3)5Co (Lig)] and [(NH3)5Ru (Lig)] (Tig = Cl, Br , I or HC2O4 ) show two consecutive one-electron reactions initiated by the formation of the metastable state In , which is then rapidly oxidized to In , and the first of which is predominating an inner-sphere mechanism. ... 

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) ... 

Finally, the apparent failure of Marcus-Hush-based theory of AF in electron transfer between Fe 20)s0rf and Fe(H20)6 is quantitatively attributable to the operation of a hydroxo-bridged (inner-sphere) mechanism," and indeed AF can serve to distinguish between outer- and inner-sphere processes in appropriate cases such as this. 

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). 

Depending on the possible interaction between substrate species and redox centers of the solid, one can consider (1) outer sphere electron transfer processes in the electrolyte-particle interface and (2) inner-sphere mechanism, involving composition changes in the catalyst and the substrate. 

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