Mechanisms for redox reactions

Let us now consider redox reactions, the other category of reactions of coordination compounds. Redox reactions are those in which the oxidation states of some atoms change. In reaction (43), the oxidation state of cobalt 

Elegant experiments that demonstrated the validity of the inner-sphere path were performed by Taube and his coworkers. Reaction (43) was one of many studied. This test tube experiment conceived by Taube on the basis of his knowledge of substitution lability of metal complexes, was the seminal work which led to his Nobel prize. It was observed that in the reduction of [CoC1(NH3)5] by Cr the chromium(IIl) product always contained a chloride ion. In more detailed studies, [CoCl(NH3)5] containing radioactive C1, was dissolved in a solution containing Cr and unlabeled CT. After the reduction, which is very rapid, the product [CrCl(H20)5] was examined and found to contain only labeled Cl . This proved that the cobalt complex was the only source of the Cf that was eventually found in the chromium(iii) complex. To explain these results, a mechanism in which the activated complex contains Co and Cr atoms linked by a chloride ion was proposed, structure (I). The chloride bridge provides a good path between the two 

Reaction (43) and similar reactions were very clever choices for this study because cobalt(iii) and chromium(iii) complexes are inert, whereas chromium(ii) and cobalt(ii) complexes are labile. Thus, the rapid redox reaction is complete long before any substitution reactions begin to occur on the cobalt(iii) and chromium(iil) complexes. The lability of [Cr(H20)6]  

Reductions of a series of cobalt(iii) complexes, [CoX(NH3)5], with chromium(II) solutions were studied. Transfer of the X group to chromium occurs when X is NCS, N3 , P04, C2H3O2, Cl, Br , and 864 (46). This suggests that all of these reactions are of the inner-sphere type. 

The rates of these tactions increased in the order C2H3O2 804 Cl Br. Presumably those ions that form bridges most readily and those that provide the best path for electrons produce the fastest reactions. 

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A compound which is a good choice for an artificial electron relay is one which can reach the reduced FADH2 active site, undergo fast electron transfer, and then transport the electrons to the electrodes as rapidly as possible. Electron-transport rate studies have been done for an enzyme electrode for glucose (G) using interdigitated array electrodes (41). The following mechanism for redox reactions in osmium polymer—GOD biosensor films has... 

Cyano complexes have been involved in many kinetic studies.1 The fast electron transfer reactions between [Fe(CN)6]3- and [Fe(CN)6]4-, and between [Mo(CN)8]3- and [Mo(CN)g]4-, for example, were important in establishing the outer-sphere mechanism for redox reactions. The kinetics of... 

Linear plots of In /c versus AG° are found in some electron-transfer reactions i hat proceed by atom transfer, but the slopes are not 0.5 as predicted for outer-sphere reactions by the Marcus theory. Care thus needs to be taken in inferring mechanisms for redox reactions from LFERs. Other LFERs, such as the I lammett and Bronsted relationships, also have been applied to redox reactions of organically complexed metal ions and similarly lead to linear plots of log/c versus E°xb (see next section). 

The bulk aqueous phase represents much of the mass of the cell. The question arises as to whether in vivo redox reactions take place in this phase. In vitro the generally accepted mechanism for redox reactions in an aqueous medium involves hydride ion transfer. This term is used to denote the passage of one proton and two electrons from substrate to coenzyme with an additional proton being released to the medium. The reduced coenzyme can then transfer the electrons and proton to an acceptor molecule, an additional proton being taken up from the medium. 

Fig. 5.4 A typical experiment used by Taube to establish the existence of an inner-sphere mechanism for redox reactions of transition metal complexes...

In the same way that we considered two limiting extremes for ligand substitution reactions, so may we distinguish two types of reaction pathway for electron transfer (or redox) reactions, as first put forth by Taube. For redox reactions, the distinction between the two mechanisms is more clearly defined, there being no continuum of reactions which follow pathways intermediate between the extremes. In one pathway, there is no covalently linked intermediate and the electron just hops from one center to the next. This is described as the outer-sphere mechanism (Fig. 9-4). 

Pourbaix diagrams for the aqueous Cd-S, Cd-Te, Cd-Se, Cu-In-Se, and Sb-S systems have been compiled and discussed by Savadogo [26] in his review regarding chemically and electrochemically deposited thin Aims for solar energy materials. Dremlyuzhenko et al. [27] analyzed theoretically the mechanisms of redox reactions in the Cdi xMn , Te and Cdi- , Zn i Te aqueous systems and evaluated the physicochemical properties of the semiconductor surfaces as a function of pH. 

In terms of gross features of mechanism, a redox reaction between transition metal complexes, having adjacent stable oxidation states, generally takes place in a simple one-equivalent change. For the post-transition and actinide elements, where there is usually a difference of two between the stable oxidation states, both single two-equivalent and consecutive one-equivalent changes are possible. 

In 1974, Olson et al. [9] proposed a mechanism for the reactions catalyzed by XO. In accord with this mechanism six electrons are transferred from fully reduced enzyme through four redox centers during the oxidation of xanthine (Reaction (1)) ... 

Further studies on this system will include IR-SEC experiments under an atmosphere of CO to verify its catalytic activity for CO reduction and to aid in formulating a mechanism for the reaction. Other multimetallic systems used as CO reduction catalysts such as mthenium-, iridium-, and cobalt-based complexes, or metal clusters used as models in the active sites of biological systems, many of which have complex redox behavior can also be investigated using the IR-SEC technique. 

Scheme 5 Above Polymerization of EDOT catalyzed by SBP using terthiophene as a redox mediator. Below Proposed mechanism for the reaction. (Reprinted with permission from Nagarajan et al. [44]. 2008, American Chemical Society)...

M. J. Weaver, Chem. Rev. 92 463 (1992). A review, oriented to mechanism determination for redox reaction. 

This chapter deals with the fundamental aspects of redox reactions in non-aque-ous solutions. In Section 4.1, we discuss solvent effects on the potentials of various types of redox couples and on reaction mechanisms. Solvent effects on redox potentials are important in connection with the electrochemical studies of such basic problems as ion solvation and electronic properties of chemical species. We then consider solvent effects on reaction kinetics, paying attention to the role of dynamical solvent properties in electron transfer processes. In Section 4.2, we deal with the potential windows in various solvents, in order to show the advantages of non-aqueous solvents as media for redox reactions. In Section 4.3, we describe some examples of practical redox titrations in non-aqueous solvents. Because many of the redox reactions are realized as electrode reactions, the subjects covered in this chapter will also appear in Part II in connection with electrochemical measurements. 

As discussed in Vol. 2, Chap. 4, experimental studies, mainly pioneered by Taube [11], revealed two different reaction pathways for redox reactions in solution (i) outer sphere mechanism characterized by weak interaction of the reactive species, with the inner coordination sphere remaining intact during the electron transfer, and reactions occurring through a common ligand shared by the metallic centers thus proceeding by an inner sphere mechanism. 

Figure 30(a) concerns the EE mechanism for the reaction O + 2 e = R. The solid curve represents the standard free energy profile pertaining to the standard potential E° of the redox couple O/R. In this case, the energy levels of the initial and the final state are equal by definition. Well... 

Estimation of rates for redox reactions in environmental systems requires that the problem be formulated in terms of specific oxidation and reduction half-reactions. In addition, we assume that the rate-limiting step of the transformation mechanism is bimolecular—that is, the slow step requires an encounter (collision) between the electron donor and electron acceptor. Under most conditions found in environmental systems, such reactions exhibit rate laws for the disappearance of a pollutant, P, that are first-order in concentration of P and first-order in the concentration of environmental oxidant or reductant, E,... 

For add-base reactions, which are fast, tables of ionization constants are directly applicable. In contrast, for redox reactions, many of which are slow, a table of electrode potentials can be no more than a guide to equilibrium conditions as it tells nothing of rates of reactions or mechanisms. For example, a table of electrode potentials leads to the expectation of a quantitative reaction between Ce(TV) and... 

Experimental investigations of pyrite formation provide important constraints on sedimentary geochemistry. Beginning with the pioneering work of Allen et al. (1912) and later that of Berner (1962, 1964b,c), the formation of pyrite at Earth surface conditions has been the subject of a number of detailed laboratory studies. Much of the earlier literature is reviewed by Morse et al. (1987). These laboratory investigations have resulted in the recognition of several potential mechanisms for formation of pyrite at T < 100 °C. All these mechanisms involve redox reactions because the ultimate source of sulfur in pyrite, H2S, is more reduced than the disulhde in pyrite. These redox reactions include ... 

Electrochemistry integrates analytical technique (determination of concentrations, reaction mechanisms, or properties9) and synthetic methods such as electrolysis.10 Electrons needed for redox reactions are provided by an electric current supplied through electrodes in a highly controlled and selective manner. Products can be isolated easier. It is well known that electrochemical redox reactions may result in reactive intermediates under mild conditions.11 Electrochemistry is a clean and convenient methodology even on the preparative scale. 

If a metal complex can be reduced by superoxide and if its reduced form can be oxidized by superoxide, both at rates competitive with superoxide disproportionation, the complex can probably act as an SOD by Mechanism I. Mechanism II has been proposed to account for the apparent catalysis of superoxide disproportionation by Lewis acidic nonredox-active metal ions under certain conditions. However, this mechanism should probably be considered possible for redox metal ions and the SOD enzymes as well. It is difficult to distinguish the two mechanisms for redox-active metal ions and the SOD enzymes unless the reduced form of the catalyst is observed directly as an intermediate in the reaction. So far it has not been possible to observe this intermediate in the SOD enzymes or the metal complexes. 

For the Cr /Cr couple, Ff = —0.41 V, and Cr(II) compounds slowly liberate H2 from water, as well as undergo oxidation by O2 (see worked example 7.4). The potential diagram in Figure 21.9 shows that Cr(II) compounds are just stable with respect to disproportionation. The study of the oxidation of Cr + species has played an important role in establishing the mechanisms of redox reactions (see Chapter 25). 

When both reactants in a redox reaction are kinetically inert, electron transfer must take place by a tunnelling or outer-sphere mechanism. For a reaction such as 25.46, AG° 0, but activation energy is needed to overcome electrostatic repulsion between ions of like charge, to stretch or shorten bonds so that they are equivalent in the transition state (see below), and to alter the solvent sphere around each complex. 

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