Inner sphere reduction mechanisms, effect

The inner-sphere reductions of [Co(NH3)5(SCONHR)] and [Co(NH3)5 (OCSNHR)] by Gr involve attack at the remote oxygen and sulfur atoms, respectively, with a subsequent isomerization of the 0-bonded ehromium(III) product in the former reaction. The unusually rapid reactions of the S-bonded cobalt(III) complexes are attributed to a structural tran -effect on the Co—N bond length, reducing the reorganization energy needed to form the transition state. A kinetic study of the Cr reduction of [Co(NH3)5(pyruvate)] reveals that the rate of reduction is dependent on the nature of pyruvate ligand, with the keto form about 400 times as reactive as the hydrated form. An inner-sphere mechanism has be postulated for the Cr reduction of [Co(NH3)5(pyridine N-oxide)] on the basis of the rate and activation parameters. The outer-sphere Cr reduction of [Co(sepulchrate)] is catalyzed by halide ions, with the ion-pair formation constants for [Co(sep), estimated to be 5.5, 2.3, and 1.7 M" for Cl", Br", and I", respectively. ... 

Replacement of chloride or bromide in tra 5 -[Pt(oxalate)2X2] by iodide proceeds by reduction to [Pt"(oxalate)2] , followed by oxidative addition to give the [Pt (oxalate)2la] product. Reaction of [Pt(SCN)g] with bases similarly has as its first step formation of [Pt"(SCN)4] . The replacement of one bromide by chloride in /ra/z5 -[Pt(CN)4Br2] and /m 5-[Pt(N02)4Br2] is catalysed by [Pt(CN)4] or [Pt(N02)4] . The reactions follow a third-order rate law the mechanism is the usual inner-sphere redox mechanism of substitution. In the cyanide case there is an additional term independent of chloride concentration in the rate law, indicating a solvent-assisted path. Closely related to these systems is the oxidative addition of bromine to [Pt(CN)4] to give rm/2j-[Pt(CN)4Br2] , which has been studied in acid solution by stopped-flow techniques. Here initial fast production of tranj -[Pt(CN)4(OH2)Br] is followed by slow displacement of water by bromide, which explains the marked catalytic effect of added bromide on the overall reaction. ... 

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. 

Chromium(II) is a very effective and important reducing agent that has played a significant and historical role in the development of redox mechanisms (Chap. 5). It has a facile ability to take part in inner-sphere redox reactions (Prob. 9). The coordinated water of Cr(II) is easily replaced by the potential bridging group of the oxidant, and after intramolecular electron transfer, the Cr(III) carries the bridging group away with it and as it is an inert product, it can be easily identified. There have been many studies of the interaction of Cr(II) with Co(III) complexes (Tables 2.6 and 5.7) and with Cr(III) complexes (Table 5.8). Only a few reductions by Cr(II) are outer-sphere (Table 5.7). By contrast, Cr(edta) Ref. 69 and Cr(bpy)3 are very effective outer-sphere reductants (Table 5.7). 

An inner-sphere electron reduction has been proposed as a possible mechanism for the Fe(II)-induced decomposition of 1,2,4-trioxolanes (ozonides) (75) and (76). Benzoic acid was found to be the major product. The nucleophilic Ee(II) species attack the ozonide from the less hindered side of the electrophilic 0-0 a orbital to generate exclusively the Ee(III) oxy-complexed radical (inner-sphere electron transfer). After selective scission of the C-C bond, the resulting carbon-centred radical produced the observed product. The substituent effect determine the regioselective generation of one of the two possible Fe(III)-complexed oxy radicals. The bond scission shown will occur if R is bulkier than R. ... 

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... 

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. 

This chapter mainly focuses on the reactivity of 02 and its partially reduced forms. Over the past 5 years, oxygen isotope fractionation has been applied to a number of mechanistic problems. The experimental and computational methods developed to examine the relevant oxidation/reduction reactions are initially discussed. The use of oxygen equilibrium isotope effects as structural probes of transition metal 02 adducts will then be presented followed by a discussion of density function theory (DFT) calculations, which have been vital to their interpretation. Following this, studies of kinetic isotope effects upon defined outer-sphere and inner-sphere reactions will be described in the context of an electron transfer theory framework. The final sections will concentrate on implications for the reaction mechanisms of metalloenzymes that react with 02, 02 -, and H202 in order to illustrate the generality of the competitive isotope fractionation method. 

A mechanism involving the polarization of the ascorbate ligand by a Cu(II) central ion was proposed (138), though the involvement of Cu(I) cannot be ruled out (139). All these reactions proceed via the inner-sphere mechanism however, the copper-catalyzed reduction of superoxide boimd to a binuclear cobalt(III) complex by 2-aminoethanethiol proceeds via the outer-sphere mechanism (140). This is attributed to the effect of 2-aminoethanethiol as a hgand on the rate constant of the Cu(ll/1) electron self-exchange reaction which is suggested to proceed via the gated mechanism. 

The product of the reduction of [Co(bpy)3] by Cr, upon aerial oxidation, is a red dimeric species, postulated to have the structure [(H20)4Cr(/x-OH)2Cr(OH2)2]. This product and the stoichiometry of the reaction suggests a two-electron process, with the bpy ligand serving as a temporary bridging radical. An investigation of the Cr(II) reduction of [Co(pd)3] (pd = pentane-2,4-dione) in water/acetone mixtures reveals outer-sphere, and mono- and di-bridged ([H ] dependent) pathways.The effect of the cosolvent on the activation parameters is observed at an acetone mole fraction of 0.06, at which point its solvation of the activated complex becomes important. The reduction of [Co(en)2(dppd)] (dppd = l,3-diphenylpropane-l,3-dione) by Cr occurs by a multistep mechanism in which the first step is the formation of the [Co(en)2(dppd )] radical, which catalyzes the inner-sphere Co(III)/Cr(II) electron transfer process. " A molecular orbital study indicates that the [Co(en)2(dppd)] reduction likely involves attack of Cr " at the methine carbon of dppd, in contrast to the attack on an oxygen in the [Co(en)(pd)2] reduction. 

Studies performed with [NiL]+ and C02 as a reducing agent allow to predict the behavior of the complexes in the electrochemical reduction of CO2. The rate constant obtained are close to rate of diffusion, except in the cases where the complex present steric effect to the inner sphere reaction . The following proposed mechanism that adjusts with the rate constants and the potential (pH range where the potential of the redox couple corresponds to an exothermic reaction) is presented ... 

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... 

---