Outer-sphere mechanisms, inorganic reactions

The Marcus treatment applies to both inorganic and organic reactions, and has been particularly useful for ET reactions between metal complexes that adopt the outer-sphere mechanism. Because the coordination spheres of both participants remain intact in the transition state and products, the assumptions of the model are most often satisfied. To illustrate the treatment we shall consider a family of reactions involving partners with known EE rate constants. 

Cyano metal complexes undergo a variety of oxidation-reduction reactions. One of the most studied is the fast self-exchange reaction of the [Fe(CN)4] /" anions information from this research was instrumental in establishing the outer-sphere mechanism (see Outer-sphere Reaction) for transition metal oxidation-reduction reactions (see Electrochemistry Applications in Inorganic Chemistry). The nature... 

Micellar Efllecls on Inorganic Reactions.—Electron transfer between ferric ion and phenothiazine is inhibited by cationic micelles and accelerated by up to 10 -fold in anionic micelles of sodium lauryl sulphate. " In both cases the rate-surfactant concentration profile can be simulated accurately. Anionic micelles only cause a small effect on the reactivity of ruthenium(iii) tris(bipyridyl) with molybdenum(iv) octacyanide but accelerate the reaction between ferrous ion and tris(tetramethylphenanthroline)iron(ra). In the latter case a plot of surfactant concentration is linear with the reciprocal of the observed rate constant. Fast outer-sphere electron-transfer reactions may decrease in rate constant by up to four orders of magnitude when one of the reactants is solubilized in an anionic micelle. When this partner is neutral the inhibition is reduced somewhat by added salt, but when it is cationic the effect may be attenuated by competitive binding of Na or HsO and exclusion of reactant from the micelle. Oxidation of diethyl sulphide is catalysed by micelles of sodium lauryl sulphate containing carboxylate-ions by the mechanism shown. (Scheme 3). The rate advantage is quantitatively accounted for by the entropy term, and hexanoate is forty-fold more effective than acetate. Electron-transfer between the anionic trans-1,2-diaminocyclohexane... 

The prospects for electron transfer mechanisms clearly extend beyond inorganic chemistry into the broad regions of organometal-lic and organic systems. Pushed to these limits, adequate quantitative criteria will be needed to delineate outer-sphere from inner-sphere mechanisms. However, the extent to which theoretical studies will provide more concrete guidelines of predictive value will determine whether electron transfer processes will form the basis of reaction mechanisms into the next century. 

It has in general been the objective of many mechanistic studies dealing with inorganic electron-transfer reactions to distinguish between outer- and inner-sphere mechanisms. Along these lines high-pressure kinetic methods and the construction of reaction volume profiles have also been employed to contribute toward a better understanding of the intimate mechanisms involved in such processes. The differentiation between outer- and inner-sphere mechanisms depends... 

However, the mechanisms of conventional redox reactions and electrochemical reactions maybe quite different. Within the formalism of electron transfer theory, the electron transfer reactions at electrodes are usually of the outer-sphere type, whereas those that involve inorganic ions are often of the inner-sphere type [11]. 

The electron transfer mechanism of azurin, a well known example for this type of proteins, has been systematically studied using the chemical relaxation method and a well defined inorganic outer sphere redox couple. In parallel, the investigations of the reaction with its presumed physiological partner, cytochrome c, were pursued (7). The specificity of the interaction between azurin and cytochrome c P-551 is expressed in higher specific rates and in the control of the electron transfer equilibrium by conformational transitions of both proteins. 

The anion exchange mechanism in Table 4.3 is the analog of the reaction in Eq. 4.45, wherein the signs of the valences are reversed B symbolizes a carboxylate group (COO ) and M is replaced by a univalent, inorganic anion that forms outer-sphere complexes with protonated surface hydroxyl or amine groups. This mechanism is not observed often, possibly because of the weakness of the surface complexes involved, but it should be prominent in acidic soils whose clay fraction comprises primarily metal oxides. 

The nature and properties of metal complexes have been the subject of important research for many years and continue to intrigue some of the world s best chemists. One of the early Nobel prizes was awarded to Alfred Werner in 1913 for developing the basic concepts of coordination chemistry. The 1983 Nobel prize in chemistry was awarded to Henry Taube of Stanford University for his pioneering research on the mechanisms of inorganic oxidation-reduction reactions. He related rates of both substitution and redox reactions of metal complexes to the electronic structures of the metals, and made extensive experimental studies to test and support these relationships. His contributions are the basis for several sections in Chapter 6 and his concept of inner- and outer-sphere electron transfer is used by scientists worldwide. 

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