Electron transfer, metal exchange reactions

Figure 16.10 illustrates the results from application of ISM for the self-exchange rates in transition-metal complexes. These are at least as good as those provided by TM-1, but can also treat the anomalous case of the Co(OH2)6 system. This reveals that ISM provides an alternative view for electron-transfer self-exchange reactions in transition-metal complexes. It is a view not in terms of solvent- and solute-driven reactions, as in the formalism of Marcus, but entirely in terms of solute-driven reactions. The relevant factors are force constants, the sum of the equilibrium bond lengths of oxidised and reduced species and the electronic properties of the same bonds (bond orders). 

Yet the view that the rates of electron transfer in simple reactions are principally independent of the electrode metal (which for some time had been current in the electrochemical literature) cannot be maintained in this strict form. Many experimental data relating to the exchange current densities of reactions involving simple cations (such as Fe and Fe ) provide evidence that the electrode metal does exert a rather strong influence on the reaction rates. 

Substitution by the SN2 mechanism and -elimination by the E2 and Elcb mechanisms are not the only reactions that can occur at C(sp3)-X. Substitution can also occur at C(sp3)-X by the SRN1 mechanism, the elimination-addition mechanism, a one-electron transfer mechanism, and metal insertion and halogen-metal exchange reactions. An alkyl halide can also undergo a-elimination to give a carbene. 

Methylations with methyl iodide were observed to proceed with high yields and stereoselectivities. Longer-chain alkyl iodides failed in most attempts. Allyl bromide reacts smoothly - however, products of low enantioenrichment (see 146g) result. We explain the fact by a single electron transfer (SET) during the alkylation. The intermediate formation of a mesomerically stabilized allyl radical supports the SET pathway [89]. A solution to this problem was most recently published by Taylor and Papillon who converted a lithio carbamate into the corresponding zinc cuprate prior to allylation [90]. Studies on the stereochemistry in a few metal-exchange reactions have been published by Nakai et al. [91]. 

The widespread use of fast-reaction methods has opened up the study of new types of reaction and new chemical species, and has raised new questions about physical aspects of mechanisms. Examples of types of reaction which could not be studied without fast-reaction techniques include the following among reactions of labile metal ions, ligand substitution, solvent exchange, and electron-transfer among organic reactions, many proton-transfer... 

So far, research in this area has emphasized metal clusters, organometallics, and quantum size semiconductor clusters and superlattices. As experimental techniques become available, the dynamics of intrazeolite reactions, such as catalysis, ligand exchange, electron transfer and radical reactions, and polymerizations will be explored in more detail. As molecular sieves with ever larger pores are being discovered, the future potential to assemble and understand supramolecular structures is enormous. 

The metal exchange reaction rates, which are much higher than those of the replacement reactions, often reveal the electron-transfer mechanism. E.g., this holds for the reaction ... 

This chapter attempts to survey the studies which have been made on the various electron transfer reactions, occurring between metal ions (of the same element) in homogeneous solution. These reactions include the types known as exchange reactions... 

Let us consider the electron transfer between two rigid metal ions located some distance x from each other in the bulk of the solution. It is assumed that the inner-sphere reorganization of the donor D and acceptor A does not take place. The experiments show that the rate constants of these reactions differ by many orders of magnitude and the processes have an activated character even for identical ions D and A. The questions to be answered are Why does the electron exchange between identical ions in the solution require activation What is the reaction coordinate ... 

In this reaction, an electron is transferred from Cr2+ to Fe3+, and such reactions are usually called electron transfer or electron exchange reactions. Electron transfer reactions may also occur in cases where only one type of metal ion is involved. For example, the reaction... 

The simplest electron transfer reactions are outer sphere. The Franck-Condon principle states that during an electronic transition, electronic motion is so rapid that the metal nuclei, the metal ligands, and solvent molecules do not have time to move. In a self-exchange example,... 

The NO/NO+ and NO/NO- self-exchange rates are quite slow (42). Therefore, the kinetics of nitric oxide electron transfer reactions are strongly affected by transition metal complexes, particularly by those that are labile and redox active which can serve to promote these reactions. Although iron is the most important metal target for nitric oxide in mammalian biology, other metal centers might also react with NO. For example, both cobalt (in the form of cobalamin) (43,44) and copper (in the form of different types of copper proteins) (45) have been identified as potential NO targets. In addition, a substantial fraction of the bacterial nitrite reductases (which catalyze reduction of NO2 to NO) are copper enzymes (46). The interactions of NO with such metal centers continue to be rich for further exploration. 

In the following sections the effect of pressure on different types of electron-transfer processes is discussed systematically. Some of our work in this area was reviewed as part of a special symposium devoted to the complementarity of various experimental techniques in the study of electron-transfer reactions (124). Swaddle and Tregloan recently reviewed electrode reactions of metal complexes in solution at high pressure (125). The main emphasis in this section is on some of the most recent work that we have been involved in, dealing with long-distance electron-transfer processes involving cytochrome c. However, by way of introduction, a short discussion on the effect of pressure on self-exchange (symmetrical) and nonsymmetrical electron-transfer reactions between transition metal complexes that have been reported in the literature, is presented. 

In the example above, the electron transfer was direct, that is, the electrons were exchanged directly from the zinc metal to the cupric ions. But such a direct electron transfer doesn t allow for any useful work to be done by the electrons. Therefore, in order to use these electrons, indirect electron transfer must be done. The two half-reactions are physically separated and connected by a wire. The electrons that are lost in the oxidation half-reaction are allowed to flow through the wire to get to the reduction half-reaction. While those electrons... 

A recently proposed semiclassical model, in which an electronic transmission coefficient and a nuclear tunneling factor are introduced as corrections to the classical activated-complex expression, is described. The nuclear tunneling corrections are shown to be important only at low temperatures or when the electron transfer is very exothermic. By contrast, corrections for nonadiabaticity may be significant for most outer-sphere reactions of metal complexes. The rate constants for the Fe(H20)6 +-Fe(H20)6 +> Ru(NH3)62+-Ru(NH3)63+ and Ru(bpy)32+-Ru(bpy)33+ electron exchange reactions predicted by the semiclassical model are in very good agreement with the observed values. The implications of the model for optically-induced electron transfer in mixed-valence systems are noted. 

Fig. 8-4. (a) Electron state density D in a metal electrode and in hydrated redox particles, (b) rate constant for electron ttmneling k, and (c) exchange reaction current electron transfer in eqiiilibrium with a redox reaction sl = lower edge of an allowed band of metal electrons. [From Gerischer, I960.]... 

Fig. 8-39. Electron state density in an electrode metal, Du, a semiconductor film, Dt, hydrated redox particles, Dredox, and exchange reaction current of redox electrons, t., in electron transfer equilibrium M = exchange current at a bare metal electrode, M/F= exchange current at a thin-film-covered metal electrode.

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