Oxidation-reduction reactions inner sphere mechanism

The classic way of demonstrating a mechanism, viz. by product analysis, is not always possible, especially with slow reactions, and the use of linear free-energy relationships is now much in evidence. Sutin has developed an alternative approach to the problem - consideration of the magnitude of the catalytic effect of added anions. This effect is known for the M + reduction (M = V, Cr, or Fe) of a hard oxidant by the inner-sphere mechanism, and a particularly useful fact comes from the observation that N3 is ca. 10 times more effective than SCN in catalysing the rate. The effect of Cl and SCN on the rates of known outer-sphere reactions are given, using the reductions of [Co(NH3)8] +, [Co(en)3] +, and [Co(phen)3] + by Cr and V. At low concentrations of added anion, X , the rate law is observed to be... 

A single oxo bridge may subtend an angle between 140° and 180°, this angle being determined by steric or electronic factors (Table 3).95 103 Almost all these examples refer to the solid state, but there are also several homo- and hetero-nuclear M—O—M and M—O—M—O—M species known in solution. Often these are intermediates in, or products of, electron transfer reactions with oxide-bridging inner-sphere mechanisms. Examples include V—O—V in V(aq)2+ reduction of VO(aq)2+, and Act—O—Cr in Cr(aq)2+ reduction of UOj+ or PuOj+ a useful and extensive list of such species has been compiled. Tlie most recent examples are another V—O—V unit, this time from VO(aq)2+ and VOJ,105 and an all-actinide species containing neptunium(VI) and uranium-(VI).106 An example of a trinuclear anion of this type, with the metal in two oxidation states, is provided by (31).107... 

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... 

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. 

It has been shown that the oxidation-reduction reactions of transition metal complexes can occur through two different mechanisms (i) inner-sphere mechanism, when the two reactants share one or more ligands of their first coordination spheres in the activated complex and (ii) outer-sphere mechanism, when the first coordination spheres of the two reactants are left intact (as far as the number and kind of ligands present are concerned) in the activated complex. As mentioned before, the inner-sphere reactions cannot be faster than ligand substitution and... 

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. 

Pyridine derivatives of the type known to catalyze the outer-sphere reduction of Co(III) catalyze the reduction of [Coensby The catalysis is inhibited by U +, and the catalyst is slowly consumed (102). The catalyst, for example, isonicotinamide, is reversibly reduced by an inner-sphere reaction with the In another group of papers, it is shown that can bring about reduction by an inner-sphere mechanism involving attachment that is remote from the cobalt atom. The oxidants were dinuclear complexes of the type of 6. 

Redox reactions involving metal ions occur by two types of mechanisms inner-sphere and outer-sphere electron transfer. In inner-sphere mechanisms, the oxidant and reductant approach intimately and share a common primary hy-... 

Marcus LFER. Oxidation-reduction reactions involving metal ions occur by (wo types of mechanisms inner- and outer-sphere electron transfer. In the former, the oxidant and reductant approach intimately and share a common primary hydration sphere so that the activated complex has a bridging ligand between the two metal ions (M—L—M ). Inner-sphere redox reactions thus involve bond forming and breaking processes like other group transfer and substitution rcaclions, and transition-state theory applies directly to them. In outer-sphere electron transfer, the primary hydration spheres remain intact. The... 

The oxidant can also dictate an outer-sphere mechanism. In Table 12.15, [Co(NH3)6] and [Co(en)3] " participate in outer-sphere electron transfer their ligands have no accessible lone pairs with which to form bonds to bridge with the reductant. The electron transfer mechanisms for the other reactions are less certain, although labile CT (aq) is assumed to react by inner-sphere mechanisms when bridging is possible. 

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. ... 

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