Reversibility, chemical

Square planar Ni11 complexes (50a) and (50b) of the quinoxaline-2,3-dithiolate ligand are oxidizable in chemically reversible, electrochemically quasi-reversible processes to yield Ni111 species, also featuring the (dxy)1 configuration.198 Interestingly, the difference in protonation state makes for a 0.20V difference in oxidation potential ((50a) +0.12V (50b) +0.32V vs. SCE), consistent with the less basic S-donors in the thione form. 

The electroactive units in the dendrimers that we are going to discuss are the metal-based moieties. An important requirement for any kind of application is the chemical redox reversibility of such moieties. The most common metal complexes able to exhibit a chemically reversible redox behavior are ferrocene and its derivatives and the iron, ruthenium and osmium complexes of polypyridine ligands. Therefore it is not surprising that most of the investigated dendrimers contain such metal-based moieties. In the electrochemical window accessible in the usual solvents (around +2/-2V) ferrocene-type complexes undergo only one redox process, whereas iron, ruthenium and osmium polypyridine complexes undergo a metal-based oxidation process and at least three ligand-based reduction processes. 

Chemical reversibility. This is the common term for a reaction which can be run in two opposite directions. However, reversibility in connection with CV refers to the conditions of the CV experiment which is diffusion-controlled and dependent on scan rate. A reaction can be irreversible with respect to the time domain of the CV test yet still be chemically reversible. 

In 1996, two reports appeared (202, 211) where it was shown for the first time that a (phenolato)copper(II) complex can be electrochemically or chemically reversibly converted to a relatively stable (phenoxyl)copper(II) species in CH2C12 or MeCN solution. Halfen et al. (202) reported the synthesis and characterization of four square-base pyramidal (phenolato)copper(II) precursors using the pendent... 

FIGURE 1.4. Cyclic voltammetry of a Nernstian System involving the reduction of free-moving molecules. Demonstration of chemical reversibility. 

A commonly measured quantity is the ratio of the anodic current at the end of the backward step to the cathodic current at the end of the first potential step. In the case of complete chemical reversibility,... 

This is the reason that the degree of chemical reversibility is often expressed as... 

Separate determination of ° and k+ required additional high-scan-rate experiments able to reach the chemical reversibility of a system of the type shown in Figure 2.4a, even if the anodic and cathodic peaks are usually more distant from each other, due to the interference of electron transfer kinetics (see Figure 1.19), and possibly, ohmic drop. 

Buston JEH, Marken F, Anderson HL (2001) Enhanced chemical reversibility of redox processes in cyanine dye rotaxanes. Chem Commun 11 1046—1047... 

In this review, wherever electrochemistry is concerned, the reversibility of a reaction refers firstly to the chemical reversibility. It also requires that the electron transfer reaction occurs at such a rate that the rate of the whole electrodic process, which is measured by the output current of the electrode, is controlled by the diffusion of the redox species towards the electrode surface. Furthermore, the surface concentrations of O and R at a given potential should be governed by the Nemst equation. 

For any process the ratio between the current of the reverse peak and that of the forward peak, /pr//pf, is particularly important. This current ratio is the parameter which allows one to judge the chemical reversibility of an electrode reaction. In fact, when such a ratio is equal to 1, the electrogenerated species Red is stable (at least on the cyclic voltammetric timescale). The /pr//pf ratio is easily calculated in computerized instrumentation or must be determined graphically in... 

It is important to underline finally that quasireversibility is an electrochemical criterion and it does not means partial chemical reversibility . 

It should be kept in mind that controlled potential electrolysis is indispensable in ascertaining the stability of every electrogenerated species (i.e. to determine the chemical reversibility of a redox process). Such a determination simply requires the recording of a cyclic voltammogram on the exhaustively electrolysed solution. The chemical reversibility implies that such a response must be quite complementary to that initially recorded. For instance, if the voltammetric profile of Figure 46 had been obtained before electrolysis, one must obtain a complementary response of the type illustrated in Figure 47 after electrolysis. 

Given the one electron nature and chemical reversibility of the oxidation, let us pass to the cyclic voltammetric analysis of the process, Table 1. 

Firstly, let us discuss its electrochemical behaviour. As previously illustrated in Chapter 2, Figure 5, the anodic response in dichlorome-thane solution also shows features of chemical reversibility 0pCApa= 1)-The peak-to-peak separation (A p = 76mV) again indicates a slight deviation from the theoretical value of 59 mV expected for an electrochemically reversible one-electron process. 

Two successive, closely spaced oxidation processes are observed. By exhaustive electrolysis, the overall process proves to be a chemically reversible two-electron process. The separation of the two processes... 

As illustrated in Figure 19, the compound displays in CH2C12 solution a single oxidation process (the small return peak marked by an asterisk is due to a slight adsorption of the oxidation product at the electrode surface). Controlled potential electrolysis shows that such an oxidation is chemically reversible and involves a two-electron step.28... 

In fact, this molecule displays a single three-electron process characterized by chemical reversibility (even if slight adsorption phenomena make the reverse peak slightly higher than the forward peak). 

As seen the complex displays either a single ferrocenyl-centred oxidation (which however looks like it is partially chemically reversible) or a single cobaltocenium-centred reduction (which is affected by electrode adsorption), thus testifying that no interaction exists among the different metallocene units. 

Despite not being electronically saturated, vanadocene is able to lose two further electrons through two distinct oxidation processes corresponding to Vn/Vm ( o/ = -0.55 V, vs. SCE) and Vm/Viv (Ep = + 0.59 V), the first of which is chemically reversible, Figure 42.74... 

This shows that vanadocene maintains its molecular structure essentially unaltered on passing to 14 valence electrons. One might expect vanadocene to have a high predisposition to accept electrons in order to achieve the stability of 18 valence electrons. Nevertheless, it displays only a single, chemically reversible, reduction process at very negative potential values (E° = -2.74 V). 

In a similar manner to manganocene, cobaltocene is easily oxidized to the corresponding cobaltocenium ion [Co(f/5-C5H5)2]+, which, as illustrated in Figure 48, exhibits two distinct chemically reversible reductions corresponding to Coni/Con and Con/CoI.88... 

As far as the redox aptitude is concerned, two separate chemically reversible oxidations are evident corresponding to the Nin/Nim and Nini/NiIV electron transfers. The relative potentials are summarized in Table 12, together with those for the permethylated derivative. 

In acetone solution [Ni2( 5 5-Ci0H8)2] undergoes two distinct oxidations with characteristics of chemical reversibility, which correspond to Ni2II,II/Ni211,111 ( ° =-0.29 V) and Ni2IUII/Ni2in in... 

These 3d1 complexes display a quasireversible one-electron oxidation corresponding to the chemically reversible step [VOivL]/[VOvL] + (L = Schiff base ligand). As an example, Figure 3 shows the cyclic voltammetric response of [VO(acen)].7 The relative potential values are reported in Table 3. 

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