Electron transfer, inner sphere

Role of the bridging ligand in inner-sphere electron transfer reactions. A. Haim, Acc. Chem. Res., 1975, 8, 264-272 (80). 

These present an interesting dichotomy in their reductions by tm(l,10-phen-anthroline)iron(ri) (ferroin) °. That of CIO2 to CIOJ is rapid, is first-order in each component ki = 1.86 0.13 l.mole sec at 35 °C) and is independent of acidity. Ferriin is the immediate product and an outer sphere electron-transfer is proposed. The reduction of CIO2 is much slower, proceeding at the same rate as dissociation of ferroin at high chlorite concentrations and a major product is feriin dimer, possibly [(phen)2Fe-0-Fe(phen)2] . Clearly the reaction depends on ligand-displacement followed by an inner-sphere electron transfer. 

Impedance Spectroscopy, 21-22 Inner sphere electron transfer, 47-48 Ion transfer reaction, 39-40... 

Electron transfer from a macrocycle (82), based on cyclen, complex to coordinated riboflavin proceeds via an inner sphere electron transfer pathway. The riboflavin coordinates through the imide and the relevance to the interception of biological electron transfer pathways is discussed.709... 

Inner-sphere electron transfers involve the inner coordination sphere of the metal complexes and usually take place through a bridging ligand. The classic example, typical of those studied and explained by H. Taube,12 is illustrated by Figure 1.11 s... 

Figure 1.11 An inner-sphere electron transfer reaction sequence. (Adapted from reference 7.)...

The authors conclude that superoxide ion probably binds in a similar fashion to the azide and that conserved water ligands in the enzyme structure both hydrogen-bond with and help guide the substrates toward the copper ion. If this is the case, then superoxide binds directly to Cu(II) (inner-sphere electron transfer) in the following reaction ... 

Inner-sphere electron-transfer reactions are not expected to obey the Butler-Volmer equation. In these reactions the breaking or formation of a bond, or an adsorption step, may be rate determining. When the reactant is adsorbed on the metal surface, the electrostatic potential that it experiences must change appreciably when the electrode potential is varied. 

As an example, we review a SERS study of the inner-sphere electron-transfer reaction of the Os(NH3)5Py(II)/(III) couple (Py stands for pyridine) adsorbed on a roughened Ag electrode [5]. Figure 15.5 shows the Raman bands for the pyridine breathing mode at several electrode potentials. At high potentials only the peak at 1020 cm-1 of the Os(III) complex is visible. As the potential is decreased, the intensity of this band is diminished, and a new peak at 992 cm"1 appears, which corresponds to the breathing mode of the Os(II) complex. However, SERS... 

B. Structural Models of dcr/Acr Inner-Sphere Electron-Transfer Transition States... 

It has long been assumed that the rates of inner-sphere electron-transfer reactions for transition-metal complexes should be sensitive to the nature of the donor and acceptor orbital symmetries. Efforts to... 

Iron(III)-catalyzed autoxidation of ascorbic acid has received considerably less attention than the comparable reactions with copper species. Anaerobic studies confirmed that Fe(III) can easily oxidize ascorbic acid to dehydroascorbic acid. Xu and Jordan reported two-stage kinetics for this system in the presence of an excess of the metal ion, and suggested the fast formation of iron(III) ascorbate complexes which undergo reversible electron transfer steps (21). However, Bansch and coworkers did not find spectral evidence for the formation of ascorbate complexes in excess ascorbic acid (22). On the basis of a combined pH, temperature and pressure dependence study these authors confirmed that the oxidation by Fe(H20)g+ proceeds via an outer-sphere mechanism, while the reaction with Fe(H20)50H2+ is substitution-controlled and follows an inner-sphere electron transfer path. To some extent, these results may contradict with the model proposed by Taqui Khan and Martell (6), because the oxidation by the metal ion may take place before the ternary oxygen complex is actually formed in Eq. (17). 

Electrochemical reactions can be broken down into two groups outer-sphere electron-transfer reactions and inner-sphere electron transfer reactions. Outer-sphere reactions are reactions that only involve electron transfer. There is no adsorption and no breaking or forming of chemical bonds. Because of their simplicity, numerous studies have been performed, many entirely theoretical.18-25 By definition, though, electrode reactions are not outer-sphere reactions. However, if charge transfer is rate limiting for an electrode reaction, it typically takes a form similar to that of an outer-sphere reaction, which is described later in this section. 

According to Taube, the inner sphere mechanism can takes place when both oxidizing and reducing agents are substitution inert and when ligand transfer from oxidant to reductant is accompanied by electron transfer. The inner sphere electron transfer mechanism may be represented by the scheme... 

The electronic structure of oxidant and reductant, nature of bridging ligand, formation as well as fission of complex are the factors which can effect the rate of the inner sphere electron transfer mechanism. 

Chemoselectivity describes the preferred formation of one out of several products due to the selective interaction of a reagent with the substrate. In electroorganic conversion, the electrode is the reagent that can influence the reaction course in several ways The electrode material can form immobilized organometallics or oxides that can shift the conversion like mediators from an outer sphere electron transfer to a more selective inner sphere electron transfer [134]. Overvoltages can suppress the hydrogen evolution in cathodic reduction... 

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