Inner-sphere mechanisms, inorganic

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... 

Since most inorganic redox processes take place between two metal complexes, they are classified according to the behaviour of the inner (first) co-ordination spheres (shells) in the transition state (Basolo and Pearson, 1967). In the transition state of an outer-sphere mechanism the inner co-ordination spheres of both metal ions are intact, i.e., no ligand to metal bond is broken or formed, whereas in the transition state of an inner-sphere mechanism a bridging ligand,... 

Solutions of indium (I) can be prepared by treatment of indium amalgam with silver triflate in dry acetonitrile in the absence of oxygen, and then diluted with water to give the low-concentration aqueous solution, which plays a sizable role in the study of the details of intermolecular electron transfer processes in solution. Aqueous In(I) solution has been used to examine the behavior of this hypovalent center in inorganic redox transformations. Reactions with complexes of the type [(NH3)5Co (Lig)] and [(NH3)5Ru (Lig)] (Tig = Cl, Br , I or HC2O4 ) show two consecutive one-electron reactions initiated by the formation of the metastable state In , which is then rapidly oxidized to In , and the first of which is predominating an inner-sphere mechanism. ... 

Copper ions catalyze a variety of inorganic redox reactions. The kinetics and mechanisms of some of these reactions were analyzed in detail. Thus, the reduction of V(IV) by Sn(II) and Ge(II) is catalyzed by Cu ions in the presence of high concentrations of Cl. The mechanism involves the reduction of Cu(II) by Sn(II) or by Ge(II) followed by the reduction of V(IV) by Cu(I) (134). These reactions proceed via the inner-sphere mechanism (134). Also the copper-catalyzed reduction of peroxonitrite by sulfite (135), the copper-catalyzed reduction of a Ni(IV) complex by thiols (136), and the reduction of superoxide boimd to binuclear cobalt(III) complexes by thiols (137) and by ascorbate (138) follow analogous inner-sphere mechanisms. Copper ions also catalyze the reduction of peroxide-boimd Cr(IV) by ascorbate(i39). 

In this chapter the reactions between metal ions in a high oxidation state and inorganic and organic substrates are discussed in detail. Many mechanistic data have been derived from these investigations, and it is now clear that many of these reactions take place via an inner-sphere mechanism with, in some cases, evidence for the formation of well-characterised transient intermediates. Inner-sphere complex formation is more likely to take place where neutral or negatively charged substrates are involved rather than with cationic reductants, and three cases of the mechanism ... 

Finally in this introductory section it seems fitting to mention Henry Taube s contribution to the 150th Anniversary celebrations of the Royal Society of Chemistry, in which he reviews the early studies in the field of reactivity of inorganic complexes, with particular reference to the key role of substitution reactivities in his unequivocal demonstration of the inner-sphere mechanism for electron transfer. 

Despite intense study of the chemical reactivity of the inorganic NO donor SNP with a number of electrophiles and nucleophiles (in particular thiols), the mechanism of NO release from this drug also remains incompletely understood. In biological systems, both enzymatic and non-enzymatic pathways appear to be involved [28]. Nitric oxide release is thought to be preceded by a one-electron reduction step followed by release of cyanide, and an inner-sphere charge transfer reaction between the ni-trosonium ion (NO+) and the ferrous iron (Fe2+). Upon addition of SNP to tissues, formation of iron nitrosyl complexes, which are in equilibrium with S-nitrosothiols, has been observed. A membrane-bound enzyme may be involved in the generation of NO from SNP in vascular tissue [35], but the exact nature of this reducing activity is unknown. 

Although OH reacts at near-diffusion-controlled rates with inorganic anions [59], there seems to bean upper limit of ca. 3 x 10 dm mol sec in the case of simple hydrated metal ions, irrespective of the reduction potential of M"". Also, there is no correlation between the measured values of 43 and the rates of exchange of water molecules in the first hydration shell of, which rules out direct substitution of OH for H2O as a general mechanism. Other mechanisms that have been proposed are (i) abstraction of H from a coordinated H2O [75,76], and (ii) OH entering the first hydration shell to increase the coordination number by one, followed by inner-sphere electron transfer [77,78]. Data reported [78] for M" = Cr, for which the half-life for water exchange is of the order of days, are consistent with mechanism (ii) ... 

In terms of the development of an understanding of the reactivity patterns of inorganic complexes, the two metals which have been pivotal are platinum and cobalt. This importance is to a large part a consequence of each metal having available one or more oxidation states which are kinetically inert. Platinum is a particularly useful element of this pair because it has two kinetically inert sets of complexes (divalent and tetravalent) in addition to the complexes of platinum(O), which is a kinetically labile center. The complexes of divalent and tetravalent platinum show significant differences. Divalent platinum forms four-coordinate planar complexes which have a coordinately unsaturated 16-electron d8 platinum center, whereas tetravalent platinum is an 18-electron d6 center which is coordinately saturated in its usual hexacoordination. In terms of mechanistic interpretation one must therefore consider both associative and dissociative substitution pathways, in addition to mechanisms involving electron transfer or inner-sphere atom transfer redox processes. A number of books and articles have been written about replacement reactions in platinum complexes, and a number of these are summarized in Table 13. 

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]. 

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