Inner-sphere mechanism surface

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. 

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). 

The fractionation mechanism is not entirely clear. Mo04 may adsorb directly to Mn-oxide surfaces by an inner-sphere mechanism in which Mn-O-Mo bonds are formed (Barling and Anbar 2004) ... 

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 18 O-tracer studies of Gordon and Taube (1962) on the oxidation of U(IV) on Pb02 have shown that both oxygen ions in the product U02 + are derived from the oxide lattice. This result indicates an inner-sphere mechanism and is compatible with a binuclear U(IV) surface complex ... 

The latter two reactions proceed via the inner-sphere mechanism (see below), that is, they require access of the substrate to the central Cu(I) ion. The disproportionation reaction requires the contact of the central copper ion with a smface, preferably a Cu°(s) surface, as the formation of a Cu° atom is extremely endothermic due to the lattice energy of copper, - 301.4 kJmol (5). Thus ligands that block sterically the approach of a substrate or of a surface to the central copper ion stabilize it (19). An extreme example is 1,4,5,7.7,8,11,12,14,14-decamethyl-l,4,8,ll-tetraazacyclotetradecane, (27). Thus [Cu(I)L ] is stable even in aerated aqueous solutions (27). In analogy, some enzymes with Cud) as the active site, for example, CuSOD, inhibit disproportionation or the reaction with O2 by inhibiting the approach of two Cu(I) central ions to each other which is required for these reactions which are thermodynamically exothermic. 

The inertness of the surface raises interesting questions. The aqueous solvent window is pushed out as a result of water electrolysis being an inner-sphere mechanism. As a result, it is often stated in the literature that BDD can detect species which other electrodes cannot due to the extended solvent window. This is certainly true of outer-sphere species, but care must be taken when considering inner-sphere species. Heterogeneous ET will be retarded for many of these species on BDD, as there are no favorable adsorption sites, pushing out their electrochemical detection potential. Therefore, each species should be considered on a case-by-case basis, in combination with the effect of surface termination. For example, both oxidation [89] and reduction, in... 

Nanostructured surfaces induce specific interactions with the analyte that, in the case of non-reversible charge transfers occurring via inner sphere mechanism, may induce lowering of the required overvoltage for the process to occur and, consequently, even lead to higher and sharper current peaks at less extreme potential values (Fig. 6.20). 

Another significant point is that adsorption to BDD is often very weak or even non-existent, and, consequently, any electron transfer which usually involves adsorption (i.e. an inner sphere mechanism) will exhibit low rates of electron transfer. A final point should be made that the BDD surface may be modified to be either hydrogen or oxygen terminated. An example of how... 

The mechanism given is in support of the existence of inner-sphere surface complexes it illustrates that one of the water molecules coordinated to the metal ion has to dissociate in order to form an inner-sphere complex if this H20-loss is slow, then the adsorption, i.e., the binding of the metal ion to the surface ligands, is slow. 

The first two pathways (a) and (b) show, respectively, the influence of H+ and of surface complex forming ligands on the non-reductive dissolution. These pathways were discussed in Chapter 5. Reductive dissolution mechanisms are illustrated in pathways (c) - (e) (Fig. 9.3). Reductants adsorbed to the hydrous oxide surface can readily exchange electrons with an Fe(III) surface center. Those reductants, such as ascorbate, that form inner-sphere surface complexes are especially efficient. The electron transfer leads to an oxidized reactant (often a radical) and a surface Fe(II) atom. The Fe(II)-0 bond in the surface of the crystalline lattice is more labile than the Fe(III)-0 bond and thus, the reduced metal center is more easily detached from the surface than the original oxidized metal center (see Eqs. 9.4a - 9.4c). 

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... 

The Electron Transfer Step. Inner-sphere and outer-sphere mechanisms of reductive dissolution are, in practice, difficult to distinguish. Rates of ligand substitution at tervalent and tetravalent metal oxide surface sites, which could be used to estimate upward limits on rates of inner-sphere reaction, are not known to any level of certainty. 

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. 

The application of surface-enhanced Raman spectroscopy (SERS) for monitoring redox and other processes at metal-solution interfaces is illustrated by means of some recent results obtained in our laboratory. The detection of adsorbed species present at outer- as well as inner-sphere reaction sites is noted. The influence of surface interaction effects on the SER spectra of adsorbed redox couples is discussed with a view towards utilizing the frequency-potential dependence of oxidation-state sensitive vibrational modes as a criterion of reactant-surface electronic coupling effects. Illustrative data are presented for Ru(NH3)63+/2+ adsorbed electrostatically to chloride-coated silver, and Fe(CN)63 /" bound to gold electrodes the latter couple appears to be valence delocalized under some conditions. The use of coupled SERS-rotating disk voltammetry measurements to examine the kinetics and mechanisms of irreversible and multistep electrochemical reactions is also discussed. Examples given are the outer- and inner-sphere one-electron reductions of Co(III) and Cr(III) complexes at silver, and the oxidation of carbon monoxide and iodide at gold electrodes. 

Figure 8.1. Reduction of tervalent metal oxide surface sites by phenol (HA) showing inner-sphere and outer-sphere mechanisms. [From Stone (1986), with permission.]...

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