Electron-transfer reactions inner-sphere

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

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

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... 

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... 

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. 

Nitroprusside is capable of ligating redox partners through a cyano ligand to participate in inner-sphere electron-transfer reactions. With hemoglobin an electron is transferred in this fashion from the porphyrin center to yield [Fe(CN)4(NO)] , methemoglobin and NO (see below) [92]. 

Such problems are frequently encountered if metal complexes are used as redox catalysts for inner-sphere electron transfer reactions (Sect. 2.3 mechanism B). 

A variety of linkage isomer pairs have been produced from somewhat more complex ligands, such as substituted pyridines and benzoic acids, for example (5a) and (5b).77,78 These complexes have been employed in detailed studies of inner-sphere electron transfer reactions in order to assess the importance of the nature and orientation of the bridge between redox centres on intramolecular electron transfer rates.77-80... 

Ito T, Shinohara H, Hatta H, Nishimoto S-l (1999) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers product and laser flash photolysis studies on the oxidative splitting mechanism. J Phys Chem A 103 8413-8420 ItoT, Shinohara H, Hatta H, Fujita S-l, Nishimoto S-l (2000) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers. 2. Conformational effects on the reductive splitting mechanism. J Phys Chem A 104 2886-2893 ItoT, Shinohara H, Hatta H, Nishimoto S-l (2002) Stereoisomeric C5-C5 -linked dehydrothymine dimers produced by radiolytic one-electron reduction of thymine derivatives in anoxic solution structural characteristics in reference to cyclobutane photodimers. J Org Chem 64 5100-5108 Jagannadham V, Steenken S (1984) One-electron reduction of nitrobenzenes by a-hydroxyalkyl radicals via addition/elimination. An example of an organic inner-sphere electron-transfer reaction. J Am Chem Soc 106 6542-6551... 

An interesting subset of the inner-sphere electron transfer reactions involves the irreversible formation of a stable metal-02 adduct as the product. A series of such reactions (Figure 9.4) has been investigated by reacting d8 and d10 organometallic complexes with O2.45 These reactions result in the formation of structurally defined side-on peroxide complexes. 

Halpern, J. and S. Nakamura Transient Intermediates in Inner-Sphere Electron-Transfer Reactions Involving Polyatomic Bridging Ligands. J. Am. Chem. Soc. 87, 3002 (1965). 

When reactions involve bonds rearrangement, or adsorption, the reacting species looses a part of its solvation shell and moves close to the electrode surface. They are called inner sphere electron transfer reactions and the electronic interactions with the electrode can be either weak or strong. Depending on the elec-... 

As noted above, outer-sphere electron transfer reactions are characterized by the absence of strong electronic interaction (e.g., bond formation) between atomic or molecular orbitals populated, in the donor and acceptor, by the transferred electron. Nonetheless, as can be appreciated intuitively, outer-sphere reactions must require some type of electronic communication between donor and acceptor atomic or molecular orbitals. This is referred to in the literature as coupling, electronic interaction, or electronic overlap and is usually less than 1 kcal/mol. Inner-sphere electron transfer reactions, by contrast, frequently involve covalent bond... 

Give an example of a reaction that proceeds by an inner-sphere mechanism. Sketch reaction profiles for inner-sphere electron-transfer reactions in which the ratedetermining step is (a) bridge formation, (b) electron transfer and (c) bridge cleavage. Which profile is most commonly observed ... 

The effect of pressure on a number of inner-sphere electron transfer reactions has also been investigated. By way of example, the reaction of Co(NH3)5X2-" with Fe(H20)6 -" exhibits Avalues of +10.7 (X = F), +8.7 (X = Cl), +6.4 (X = Br), and +13.0 (X = N3 ), which are mainly ascribed to the release of a solvent molecule during the formation of the bridged inner-sphere species, [(NH3)5Co-X-Fe(H20)5] (24). Other examples of pressure effects on inner-sphere electron transfer reactions, also including some intramolecular reactions induced by pulse radiolysis, have been reported in the htera-ture (i, 25, 26). 

V. Jagannadham and S. Steenken, One-electron reduction of nitrobenzenes by a-hydroxyalkyl radicals via addition/elimination. An example of an organic inner-sphere electron-transfer reaction, J. Am. Chem. Soc., 106 (1984) 6542. 

The M(NH3)5X"" systems (M = Co, Cr) have particular utility in providing many examples for the study of substitution and inner-sphere electron transfer reactions (Ch ter 7.2) and recent work has shown that the Co(NHj)5 group has excellent potential as a protecting group in the synthesis of peptides. The ammine chemistry of Co", 11.12,15 Ryiu 104 105,106 QgU 105 Group... 

The reactions of both trisoxalato ferrate(III) and trisoxalatocobaltate-(III) with ascorbic acid have been examined. Evidence is presented in the reaction with Fe(III) for the formation of an intermediate prior to a relatively slow inner-sphere electron transfer reaction, which is independent of pH, ascorbic acid concentration, and ionic strength. For the cobalt complex, the pH (8-10) of the reaction medium leads to reaction with the doubly deprotonated ascorbate ion for which the rate constant is 20 s at 25 °C. Complexation of Fe(III) by... 

The redox reactions at metal electrodes can be treated in a similar way. Use of the oscillator model for the solvent implies that the solvent reorganization takes place at a fixed separation of the ion from the metal surface. With this condition all results obtained in Sec.3 2.IV for homogeneous outer-sphere and inner-sphere electron-transfer reactions can be directly applied by relating the current density (per unit ion concentration) to the expressions for the rate constant through equation (106.IV). 

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