Polarography and Voltammetry of Metal Complexes

Some redox couples of organometallic complexes are used as potential references. In particular, the ferrocenium ion/ferrocene (Fc+/Fc) and bis(biphenyl)chromium(I)/ (0) (BCr+/BCr) couples have been recommended by IUPAC as the potential reference in each individual solvent (Section 6.1.3) [11]. Furthermore, these couples are often used as solvent-independent potential references for comparing the potentials in different solvents [21]. The oxidized and reduced forms of each couple have similar structures and large sizes. Moreover, the positive charge in the oxidized form is surrounded by bulky ligands. Thus, the potentials of these redox couples are expected to be fairly free of the effects of solvents and reactive impurities. However, these couples do have some problems. One problem is that in aqueous solutions Fc+ in water behaves somewhat differently to in other solvents [29] the solubility of BCr+BPhF is insufficient in aqueous solutions, although it increases somewhat at higher temperatures ( 45°C) [22]. The other problem is that the potentials of these couples are influenced to some extent by solvent permittivity this was discussed in 8 of Chapter 2. The influence of solvent permittivity can be removed by 

5) The dynamical solvent effects on the kinetics of electron-transfer processes have been reviewed in detail in Refs [24a] and [32] and concisely in Section 3.6 of Ref. [8]. 

The ligand of a metal complex and the solvent molecule compete with each other, as Lewis bases, to interact with the central metal ion. At the same time, the metal ion and the solvent molecule compete with each other, as Lewis acids, to interact with the ligand. Thus, the behavior of a metal complex is easily influenced by the Lewis acid-base properties of solvents. In an aprotic solvent, which is of weak acid- 

This so-called ligand relaxation process is detected by the positive shift of the po-larographic reduction wave of Fe(acac)3. If the cation of the supporting electrolyte is R4N+, which is a very weak Lewis acid, this process does not occur. 

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