Redox Reactions in Non-Aqueous Solvents

Like metal chelates of some other dithio ligands, e.g. maleonitrile dithiolate, dithio-/J-diketonato complexes undergo reversible redox reactions in non-aqueous solvents. The square-planar complexes [ML2] (M = Co, Ni, Pd and Pt) exhibit two consecutive one-electron reductions as shown in equation (5).252,253... 

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. 

The characteristics of redox reactions in non-aqueous solutions were discussed in Chapter 4. Potentiometry is a powerful tool for studying redox reactions, although polarography and voltammetry are more popular. The indicator electrode is a platinum wire or other inert electrode. We can accurately determine the standard potential of a redox couple by measuring the electrode potential in the solution containing both the reduced and the oxidized forms of known concentrations. Poten-tiometric redox titrations are also useful to elucidate redox reaction mechanisms and to obtain standard redox potentials. In some solvents, the measurable potential range is much wider than in aqueous solutions and various redox reactions that are impossible in aqueous solutions are possible. 

Haim and Taube reacted [CoN3(NH3)s]2+ with nitrosyl chloride in water S which led to the formation of [Co(NH3)5(H20)]3+, dinitrogen, and nitrous oxide. [Co(N3)Cl(en)2]+ underwent a similar reaction in water56 57 with formation of [CoCl(en)2(H20)]2+. Following this early work, several methods were developed that similarly employed azide ligand elimination as the result of redox transformations in non-aqueous solvents. The complex [CoN3(NH3)s]2+ and nitrosonium salts were reacted in trimethylphosphate, tetramethylene sulfone,59 or organonitriles with the formation of the... 

Ligand induced redox reactions of low oxidation state rhenium halides and related systems in non-aqueous solvents. R. A. Walton, Prog. Inorg. Chem., 1976, 21, 105-127 181). 

Added stability in PEC can be attained through the use of non-aqueous solvents. Noufi et al. [68] systematically evaluated various non-aqueous ferro-ferricyanide electrolytes (DMF, acetonitrile, PC, alcohols) for use in stabilizing n-CdSe photoanodes. Selection of the solvent was discussed in terms of inherent stability provided, the rate of the redox reaction, the tendency toward specific adsorption of the redox species, and the formal potential of the redox couple with respect to the flat band potential (attainable open-circuit voltage). On the basis of these data, the methanol/Fe(CN)6 system (transparent below 2.6 eV) was chosen as providing complete stabilization of CdSe. Results were presented for cells of the type... 

This book was written to provide readers with some knowledge of electrochemistry in non-aqueous solutions, from its fundamentals to the latest developments, including the current situation concerning hazardous solvents. The book is divided into two parts. Part I (Chapters 1 to 4) contains a discussion of solvent properties and then deals with solvent effects on chemical processes such as ion solvation, ion complexation, electrolyte dissociation, acid-base reactions and redox reactions. Such solvent effects are of fundamental importance in understanding chem-... 

There are, of course, many other types of reaction that can profitably be studied in solvents other than water. Redox reactions will be discussed in Section 9.4, where we will look at analogies between reduction/ oxidation and acid/base processes, and the use of non-aqueous solvents as media suitable for the use of strongly oxidising or reducing agents. 

In non-aqueous electrolytes, the different properties of the solvated metal ions lead to different equilibrium and standard potentials. For comparing standard potentials, electrode reactions should be defined as reference systems with similar values in different solvents. Koepp, Wendt, and Strehlow suggested ferrocene/ferrocinium and cobaltocene/ cobaltocenium redox systems. The redox systems are bis-pentadienyl complexes of Fe +/Fe + and Co /Co , respectively. Gritzner and Kuta recommended ferrocene/ferrocinium and bis(biphenyl)Cr(l)/bis(biphenyl)Cr(0). Salt bridges with conventional cells should be avoided. Similar to aqueous electrolytes a reference to the physical potential scale is possible. Similar considerations hold for ionic melts and molten and solid electrolytes. 

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