Metal reduction electron transfer

Fig. 11-1. Mixed electrode model (local cell model) for corrosion of metals i = anodic current for transfer of iron ions i = cathodic current of electron transfer for reduction of hydrogen ions.

The theory of electron transfer in chemical and biological systems has been discussed by Marcus and many other workers 74 84). Recently, Larson 8l) has discussed the theory of electron transfer in protein and polymer-metal complex structures on the basis of a model first proposed by Marcus. In biological systems, electrons are mediated between redox centers over large distances (1.5 to 3.0 nm). Under non-adiabatic conditions, as the two energy surfaces have little interaction (Fig. 5), the electron transfer reaction does not occur. If there is weak interaction between the two surfaces, a, and a2, the system tends to split into two continuous energy surfaces, A3 and A2, with a small gap A which corresponds to the electronic coupling matrix element. Under such conditions, electron transfer from reductant to oxidant may occur, with the probability (x) given by Eq. (10),... 

An electron can be given to a cation and the process is known as the single-electron-transfer (SET) reduction. The source of one-electron transfer is the metal ion. For example, Cu" ions are used for the decomposition of acyl peroxides (Scheme 2.32). This is a convenient method for the generation of the ArCOO radicals, especially because in thermolysis the ArCOO radicals further decompose to Ar and CO2. 

The key to the outer sphere mechanism is that electron transfer from reductant to oxidant occurs with the coordination shells (or spheres) of each reactant staying intact throughout. Since the coordination (or inner) sphere, that is the set of bound ligands, is not changed during the reactions, it appears that the key to the process lies beyond these, in the outer sphere around the reactants. We have seen an example earlier involving two different metal centres. Another classical example of pure electron transfer alone, involving two oxidation states of the one metal ion, is Equation (5.54). 

So far we have emphasized electron transfer (oxidation-reduction) reactions that involve a metal and a nonmetal. Electron transfer reactions can also take place between two nonmetals. We will not discuss these reactions in detail here. All we will say at this point is that one sure sign of an oxidation-reduction reaction between nonmetals is the presence of oxygen, 0 g), as a reactant or product. In fact, oxidation got its name from oxygen. Thus the reactions... 

Cu BSP], the preoxidized form [Cu BSP(BF4)], and and the pre-reduced form [Cu BSP] . The alcohol first coordinates to the catalyst precursor [Cu BSP], leading to the formation of a metal-phenoxyl radical complex. This species undergoes the substrate C -H proton abstraction by the radical, followed by a rapid intramolecular electron transfer with reduction of Cu to Cu. After this rate-determining step, the copper(I) species reacts with dioxygen to form an hydroperoxo copper(II) with the release of the carbonyl product. Finally, dihydrogen peroxide is replaced by a new alcohol molecule to give back the active species (Fig. 11). 

Keywords Electron transfer Metal complex photocatalyst Photocatalytic CO2 reduction Semiconductor photocatalyst Supramolecular chemistry... 

For convenience, it has been proposed that the ATRP equilibrium constant (represented in Scheme 1 as Kj trp = oJ kdeact) is expressed as a combination of four reversible reactions oxidation of the metal complex, or electron transfer (Ket), reduction of a halogen to a hahde ion, or electron affinity (Kb/, alkyl halide bond homolysis (Kbh), and association of the halide ion to the metal complex, or halogenophilicity (Xx) (Scheme... 

The actual site of electron transfer upon reduction or oxidation of porphyrins with bound NO groups has not been well established except for compounds with Fe(II) or Co(II) central metals, where an oxidation of the former complex involves the Fe(II)/Fe(III) redox couple and an oxidation of the latter involves electrogeneration of a Co(II) porphyrin rr-cation radical (7j. 

Both low and high oxidation states of nickel porphyrins are easily accessible in many nonaqueous solvents and the exact site of electron transfer (metal vs ring) upon reduction or oxidation has been the topic of numerous studies [7]. Bocian and coworkers [325], as well as Connich and Macor [326], showed that a Ni(II) porphyrin tt-cation radical can be converted to a Ni(III) porphyrin in binding solvents such as Py, THF or MeCN. The type of Ni(II) porphyrin tt-cation radical, that is, aiu or a2u, wiU depend on the type of porphyrin macrocycle anda Ni(II) jr-cation... 

This adduct can be further oxidized (e.g., by metal ions or oxygen) by a one-electron transfer (monovalent reduction), resulting in a oxidized but stable product. In the first case, the extra electron is transferred to the metal ion, and in the second case, to oxygen (forming superoxide). 

Cerium(iv).— The acid-promoted redox decomposition and cerium(iv) oxidation of the tris(oxalato)cobaltate(in) ion have been studied in aqueous acid media. In IM sulphuric acid, in the absence of oxidant, there occurs an induction period prior to the internal redox decomposition of the anion. On addition of the cerium(iv), however, there results an increased rate of reduction of the cobalt(ni) centre in contrast to the behaviour of this oxidant to M(C20 ) complexes where M = Cr, Rh, or Ir. The induction in the add-catalysed decomposition is consistent with the formation of a unidentate oxalato-complex-ion which may be the main route towards the stepwise reduction to yield Co and COg. From spectral studies on the total expected absorbance values on mixing, it would appear that the cerium(iv) ion is involved in the pre-equilibrium formation of a dinuclear species which might undergo internal electron transfer with reduction to cerium(in). A possible mechanism in this system may then be written as shown in Scheme 5 (ox = C2O4 -). The variations in rate of the one-electron redox reactions of this type are dependent on the nature of the activated complex, which may differ from one metal centre to another in respect of the number of protons and sulphate anions incorporated. 

Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

Much of tills chapter concerns ET reactions in solution. However, gas phase ET processes are well known too. See figure C3.2.1. The Tiarjioon mechanism by which halogens oxidize alkali metals is fundamentally an electron transfer reaction [2]. One might guess, from tliis simple reaction, some of tlie stmctural parameters tliat control ET rates relative electron affinities of reactants, reactant separation distance, bond lengtli changes upon oxidation/reduction, vibrational frequencies, etc. 

Using the electron transfer definition, many more reactions can be identified as redox (reduction-oxidation) reactions. An example is the displacement of a metal from its salt by a more reactive metal. Consider the reaction between zinc and a solution of copper(If) sulphate, which can be represented by the equation... 

The mechanism by which the Birch reduction of benzene takes place (Figure 118) IS analogous to the mechanism for the metal-ammonia reduction of alkynes It involves a sequence of four steps m which steps 1 and 3 are single electron transfers from the metal and steps 2 and 4 are proton transfers from the alcohol... 

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

Hydroperoxides are decomposed readily by multivalent metal ions, ie, Cu, Co, Fe, V, Mn, Sn, Pb, etc, by an oxidation-reduction or electron-transfer process. Depending on the metal and its valence state, metallic cations either donate or accept electrons when reacting with hydroperoxides (45). Either one... 

Oxidation—Reduction. Redox or oxidation—reduction reactions are often governed by the hard—soft base rule. For example, a metal in a low oxidation state (relatively soft) can be oxidized more easily if surrounded by hard ligands or a hard solvent. Metals tend toward hard-acid behavior on oxidation. Redox rates are often limited by substitution rates of the reactant so that direct electron transfer can occur (16). If substitution is very slow, an outer sphere or tunneling reaction may occur. One-electron transfers are normally favored over multielectron processes, especially when three or more species must aggregate prior to reaction. However, oxidative addition... 

The simplest electroplating baths consist of a solution of a soluble metal salt. Electrons ate suppHed to the conductive metal surface, where electron transfer to and reduction of the dissolved metal ions occur. Such simple electroplating baths ate rarely satisfactory, and additives ate requited to control conductivity, pH, crystal stmcture, throwing power, and other conditions. 

The standard electrode potentials , or the standard chemical potentials /X , may be used to calculate the free energy decrease —AG and the equilibrium constant /T of a corrosion reaction (see Appendix 20.2). Any corrosion reaction in aqueous solution must involve oxidation of the metal and reduction of a species in solution (an electron acceptor) with consequent electron transfer between the two reactants. Thus the corrosion of zinc ( In +zzn = —0-76 V) in a reducing acid of pH = 4 (a = 10 ) may be represented by the reaction ... 

The complex cyanides of transition metals, especially the iron group, are very stable in aqueous solution. Their high co-ordination numbers mean the metal core of the complex is effectively shielded, and the metal-cyanide bonds, which share electrons with unfilled inner orbitals of the metal, may have a much more covalent character. Single electron transfer to the ferri-cyanide ion as a whole is easy (reducing it to ferrocyanide, with no alteration of co-ordination), but further reduction does not occur. 

Thus, Experiment 7 involved the same oxidation-reduction reaction but the electron transfer must have occurred locally between individual copper atoms (in the metal) and individual silver ions (in the solution near the metal surface). This local transfer replaces the wire middleman in the cell, which carries electrons from one beaker (where they are released by copper) to the other (where they are accepted by silver ions). 

Thus, the reaction by which a metal dissolves in an acid is conveniently discussed in terms of oxidation and reduction involving electron transfer. The reaction can be divided into half-reactions to show the electron gain (by H+ ions) and the electron loss (by metal atoms). 

---