Outer-and Inner-Sphere Reactions

Reductions of various Co(ni) complexes by Fe(II) have been studied under high pressures . The motivation for performing such experiments resides in the possibility that the volume of activation (AF ), like the entropy of activation, might be a criterion for distinguishing between inner- and outer-sphere reactions. For reactions of the type... 

Nevertheless, the mechanism of the Shvo s catalyst has been one of the most controversial regarding the nature of the hydrogen-transfer process (84). The analysis of this reaction mechanism served as an example of comparison of both the inner- and outer-sphere reaction pathways for hydrogenation of polar, C=0 (85-87) and C=N (88—95) and unpolar bonds (95). In the next subsections are presented the mechanistic studies we carried out for the hydrogenation of ketones, imines, alkenes, and alkynes (29,87,95). 

Studies on reactions in solution expanded from equilibrium to non-equilibrium state, and the most remarkable study done in this period was that by Eigen in Germany. Thus, the northern Europe has a tradition for electrolyte solution chemistry since Arrhenius. In USA at the same time, Taubc introduced a new concept of inner- and outer-sphere reaction mechanisms. 

The rates of many electron transfer reactions have now been measured and, as new coordination complexes are prepared, characterized, and their solution properties studied, our understanding of fundamental structure (both geometric and electronic)-reactivity relationships continues to grow. Both inner- and outer-sphere reactions will be explored in Chapter 5. 

The theme here is electron transfer, in inner- and outer-sphere reactions and, to a lesser degree, in related processes like optically induced charge transfer or excited state decay. Three books have been written on electron transfer, by Reynolds and Luniry, Cannon and Ulstrup, the last of which emphasizes theoretical aspects. In addition, a series of theoretical and experimental articles appear in the book Tunneling in Biological Systems , edited by Chance et and in volume 74 (1982) of the Faraday Discussions of the Chemical Society. A number of reviews have appeared, dealing both with general aspects - and more specialized themes, and many will be referred to in the sections that follow. 

These curves can be related to the inner-and outer-sphere reactions pathways. The full line represents the outer-sphere reaction, whereas the inner-sphere reactions are depicted by the dashed and dotted Unes. As can be seen, outer-sphere reactions are always weak overlapping reactions, that is, reactions in which the electrode is not acting as an electrocatalyst. Inner-sphere reactions can be either strong or weak overlap reactions, depending on the properties of the reactant species and electrode material. From a chemical point of view, a chemical bond between the reactant species and the electrode is formed in the strong overlap cases, that is, the reactant species are chemisorbed on the electrode surface. 

The reductive elimination of vinyl acetate can then proceed through either an inner-or an outer-sphere reaction channel. The inner-sphere mechanism involves the reaction between two ligands that are both already coordinated to the metal center. The outer-sphere mechanism involves the reaction between ethylene which is coordinated to the metal center and an acetate anion which resides in solution. A sketch of both the inner-and outer-sphere reactions is given in Fig. 6.19. 

Differentiation between inner- and outer-sphere complexes may be possible on the basis of determination of activation volumes of dediazoniations catalyzed by various metal complexes, similar to the differentiation between heterolytic and homolytic dediazoniations in DMSO made by Kuokkanen, 1989 (see Sec. 8.7). If outer-sphere complexes are involved in a dediazoniation, larger (positive) volumes of activation are expected than those for the comparable reactions with inner-sphere complexes. Such investigations have not been made, however, so far as we are aware. 

The complex has been separated by ion exchange and characterised by direct analysis . The complex has a distinctive absorption spectrum (Fig. 11), quite unlike that of Np(V) and Cr(III). The rate coefficient for the first-order decomposition of the complex is 2.32 x 10 sec at 25 °C in 1.0 M HCIO. Sullivan has obtained a value for the equilibrium constant of the complex, K = [Np(V) Cr(III)]/[Np(V)][Cr(III)], of 2.62 + 0.48 at 25 °C by spectrophotometric experiments. The associated thermodynamic functions are AH = —3.3 kcal. mole" and AS = —9.0 cal.deg . mole . The rates of decay and aquation of the complex, measured at 992 m/t, were investigated in detail. The same complex is formed when Np(VI) is reduced by Cr(II), and it is concluded that the latter reaction proceeds through both inner- and outer-sphere paths. It is noteworthy that the substitution-inert Rh(lII), like Cr(III), forms a complex with Np(V) °. This bright-yellow Np(V) Rh(III) dimer has been separated by ion-exchange... 

Figure 6.1 Reorganization of inner and outer sphere during an electron-transfer reaction.

A is a measure for the energy required to reorganize the inner and outer sphere during the reaction. The energy of activation for the oxidation is the saddle point energy minus the initial energy ered, which gives ... 

The exact form of the pre-exponential factor A (see Chapter 5) is still being debated from the preceding considerations it is apparent that we must distinguish two cases If the reaction is adiabatic, the pre-exponential factor will be determined solely by the dynamics of the inner and outer sphere if it is nonadiabatic, it will depend on the electronic overlap between the initial and final state, which determines the probability with which the reaction proceeds once the system is on the reaction hypersurface. 

Innumerable experiments have been performed on both inner- and outer-sphere electron-transfer reactions. We do not review them here, but present a few results that are directly relevant to the theoretical issues raised in the preceding chapters. 

When a reaction is adiabatic, the electron is transferred every time the system crosses the reaction hypersurface. In this case the preexponential factor is determined solely by the dynamics of the inner-and outer-sphere reorganization. Consequently the reaction rate is independent of the strength of the electronic interaction between the reactant and the metal. In particular, the reaction rate should be independent of the nature of the metal, which acts simply as an electron donor and acceptor. Almost by definition adiabatic electron-transfer reactions are expected to be fast. 

Complexation reactions are assumed to proceed by a mechanism that involves initial formation of a species in which the cation and the ligand (anion) are separated by one or more intervening molecules of water. The expulsion of this water leads to the formation of the inner sphere complex, in which the anion and cation are in direct contact. Some ligands cannot displace the water and complexation terminates with the formation of the outer sphere species, in which the cation and anion are separated by a molecule of water. Metal cations have been found to form stable inner and outer sphere complexes and for some ligands both forms of complexes may be present simultaneously. 

Fig. 9.17. Schematic diagram of the potential energy surfaces for an electron transfer reaction the generalized coordinates xand y correspond to inner and outer sphere modes, respectively. (Reprinted from R. J. D. Miller, G. L. McLendon, A. J. Nozik, W. Schnickle, and F. Willig, Surface Electron Transfer Processes, p. 58, copyright 1995 VCH-Wiley. Reprinted by permission of John Wiley Sons, Inc.)...

There has been some exploration of the mechanism of reduction of d transition metal complexes by M2+(aq) (M = Eu, Yb, Sm). Both inner- and outer-sphere mechanisms are believed to operate. Thus the ready reduction of [Co(en)3]3+ by Eu2+(aq) is necessarily outer-sphere. 2 However, the strong rate dependence on the nature of X when [Co(NH3)5X]2+ or [Cr(H20)5X]2+ (X = F, Cl, Br or I) are reduced by Eu2+(aq) possibly suggests an inner-sphere mechanism.653 The more vigorous reducing agent Yb2+ reacts with [Co(NH3)6]3+ and [Co(en)3]3+ by an outer-sphere route but with [Cr(H20)5X]2+ (X = halide) by the inner-sphere mechanism.654 Outer-sphere redox reactions are catalyzed by electron-transfer catalysts such as derivatives of isonicotinic acid, one of the most efficient of which is iV-phenyl-methylisonicotinate, as the free radical intermediate does not suffer attenuation through disproportionation. Using this catalyst, the outer-sphere reaction between Eu2+(aq) and [Co(py)(NH3)5]3+ proceeds as in reactions (18) and (19). Values found were ki = 5.8 x KFM-1 s 1 and k kx = 16.655... 

Oxidation of benzoin to benzil with Fe(III), in the presence of 2,2/-bipyridine or ferrozine is of first order in Fe(III) and benzoin. An inverse second-order dependence was observed with respect to hydrogen ions. For oxidation of substituted benzoins the reaction constant is p 1.2, indicating an electron-rich transition state and an inner sphere mechanism has been proposed.66 The order with respect to iodide, in the reaction of iodide ions with a diiron(III)-l,10-phenanthroline complex, is 2. The hydrolytic derivatives of the complex are not kinetically active. Both inner and outer sphere pathways are operative.67... 

However, some authors consider that an e.t. reaction which takes place within a complex of two molecules qualifies as an inner sphere process [15], The distinction between inner- and outer-sphere e.t. reactions is then blurred, and only long-range e.t. between distant molecules or chromophores would be truly outersphere... 

The most important classes of bimolecular reactions of transition metal complexes are ligand substitutions, reactions of the coordinated ligands and inner and outer sphere oxidation-reduction reactions28. ... 

Using coal-based sorbents, Sivasamy et al. [62] evaluated their ability to remove fluoride from water. On equilibrium basis, Langmuir and Freundlich models were used to describe the data points, while the kinetic data points were interpreted in terms of reaction and mass transfer processes. Kaolinite, adioctahedral two-layered (silica and alumina) silicate (1 2 type), has also been tested in drinking water defluoridation. Recently, Sugita etal. [58] and earlier Kau etal. [63] and Weerasooriya et al. [10] presented fluoride adsorption results of kaolinite. The fluoride-binding sites in kaolinite consist of aluminol and silinol sites. The authors explained that the fluoride-kaolinite interaction led to the formations of both the inner- and outer-sphere complexes. 

Relaxation of complicated ligands may occur as a step in both pathways. Diebler and Eigen 461 indicated the ways in which such mechanisms could be analysed using fast reaction methods. Several studies of Ln(III) complex formation and of the formation of Ln(III) mixed complexes have been analysed. Generally the dissociative mechanism is considered to dominate and we are then concerned with the water exchange rate. Several studies have shown that the rate decreases from La(III) to Lu(III) but there seems to be a minimum rate around Tm(III). This is also seen in the rate of rotation of ligands on the surface of the ions, Fig. 7. There may be a small crystal field term, or another contribution to a tetrad -like effect from the 4f electron core. However in the hydrate the precise relationship between the inner and outer sphere water may also be important as we saw when we discussed the heat and entropy of complex ion formation. 

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