Electrochemical and chemical reversibility

Finally, a term that should be clearly defined and one that is often used haphazardly is that of reversibility. One must make a clear distinction between electrochemical reversibility and chemical reversibility. 

This stems directly from the fact that the electron transfer kinetics for the forward and reverse processes are so facile that equilibrium is attained at each potential applied in the time-scale of the particular experiment. Thus an electron transfer may be termed electrochemically reversible at a scan rate of 50 mV s but irreversible at 1000 Vs The term is therefore a practical rather than absolute one and is dependent upon the time-scale of the electrochemical measurement. 

Chemical reversibility. This refers to the stability of the species associated in an electron transfer step to chemical decomposition. Therefore, if in (19) species B decomposes irreversibly as it is formed from the one-electron transfer process, Ae whole process would be described as being chemically irreversible. However, if the chemical step associated with the decomposition of B was sufficiently fast in both directions on the time-scale of the 

These concepts related to electrochemical and chemical reversibility can be demonstrated by considering the EC mechanism described by (19) and (27). 

Some authors would define the above processes as E , E ErQ and ErQ respectively (Bard and Faulkner, 1980). However, for simplicity, we have chosen not to use this more precise notation in this review. 

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Because of the peculiar electrochemical behavior (a single cyclic voltammetric wave characterized by remarkable electrochemical and chemical reversibility), dendrimers terminated with ferrocene-type units can be profitable used as exoreceptors, provided that they contain a group able of interacting through noncovalent bonds with the species to be recognized. Furthermore, such a group has to be located near the ferrocene units to sufficiently perturb their electrochemical response as a consequence of the interaction with the guest species. 

A redox couple in which both species rapidly exchange electrons with the working electrode is termed an electrochemically reversible couple. Such a couple can be identified from a cyclic voltammogram by measurement of the potential difference between the two peak potentials. Equation 3.25 applies to a system that is both electrochemically and chemically reversible ... 

CH2C12).94,95 The one-electron reduction is both electrochemically and chemically reversible as demonstrated by the isolation of (BuJN)3 Re(NCS)6 upon using N2H4 H20 as reducing agent (Section 43.5.1.4).96... 

At very slow scan rates the E and C steps are now reversible. This would be described as an electrochemically and chemically reversible process. 

The two major classes of voltammetric technique 4 Evaluation of reaction mechanisms 6 General concepts of voltammetry 6 Electrodes roles and experimental considerations 8 The overall electrochemical cell experimental considerations 12 Presentation of voltammetric data 14 Faradaic and non-Faradaic currents 15 Electrode processes 17 Electron transfer 22 Homogeneous chemical kinetics 22 Electrochemical and chemical reversibility 25 Cyclic voltammetry 27 A basic description 27 Simple electron-transfer processes 29 Mechanistic examples 35... 

Oxidation to the formally cobalt(lV) species [Co(R2dtc)3]+ is difficult [ E° = 0.96 V (cyclohexyl), 1.22 V (benzyl), acetone solvent) but at Pt in CHjClj it is electrochemically and chemically reversible with controlled potential electrolysis resulting in monomeric [Co(Cy2dtc)3]+ (2 ax ... 

The oxygen atom reduces the electron donating properties of the nitrogen atom which renders the complexed transition metal cation more positively charged and this leads to a reduction at less negative potential. Cyclic voltammetry showed that the reduction-oxidation process is in general both electrochemically and chemically reversible. The latter observation is important for electrocat ysis, because the electrocatalyst should be stable after electrochemical reduction (activation) before reacting with a substrate. 

The simple use of a chemically irreversible chemical reaction step representing a chemical process is physically unrealistic, because the law of microscopic reversibility or detailed balance [94] is violated. More realistic is the use of an reaction scheme (Eq. 11.1.22, Fig. 11.1.21b). Even for the relatively simple reaction scheme, interesting additional consequences arise when the possibility of reversibility of the chemical step is considered. In Fig. II. 1.2lb, cyclic voltammograms for the case of a reversible electron transfer process coupled to a chemical process with kf = 10 s and fcb = 10 s" are shown. At a scan rate of 10 mV s a well-defined electrochemically and chemically reversible voltammetric wave is found with a shift in the reversible half-wave potential E1/2 from Ef being evident due to the presence of the fast equilibrium step. The shift is AEi/2 = RT/F ln(X) = -177 mV at 298 K in the example considered. At faster scan rates the voltammetric response departs from chemical reversibility since equilibrium can no longer be maintained. The reason for this is associated with the back reaction rate of ky, = 10 s or, correspondingly, the reaction layer, Reaction = = 32 pm. At Sufficiently fast scan rates, the product B is irre-... 

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... 

An unusual reaction pattern has been found for the electrochemical and chemical (by ascorbic acid) reduction of the rhenium(II) thioether complex [Re(9S3)2] " (9S3 = 1,4,7-trithiacyclono-nane). Instead of the formation of the corresponding rhenium(I) complex, C bond cleavage and the release of ethene was observed and the brown rhenium(III) species [Re(9S3)(SC2H4SC2H4S)]" (250) was isolated as a BF4 salt. Lfpon electrochemical reduction of [Re(9S3)(SC2H4SC2H4S)]" further loss of ethene was observed while the analogous technetium complex can be reversibly reduced to [Tc(9S3)(SC2H4SC2H4S)]. 

The ligands 369 react with [RuCl2(dmso)4] to yield [RuCl2(dmso)2(369-A, 0)], characterized W spectroscopic and electrochemical methods. Complexes in the families [Ru"(bpy)(370)2] and [Ru" (aca( (370)2] have been reported. The complexes [Ru(bpy)(370)2] undergo a reversible Ru"/Ru" oxidation followed by an irreversible Ru /Ru process the bpy-centered one-electron reduction is also observed. Chemical oxidation of the complexes [Ru(bpy)(370)2] gives [Ru(bpy)(370)2] (isolated as the iodides), the electronic and ESR spectroscopic properties of which have been described. The crystal structure of [Ru(acac)(371)2] has been established, and the electrochemical and chemical redox reactions of [Ru(acac)(370)2] and [Ru(acac)(371)2] generate Ru" and Ru species that have been characterized by spectroscopic and electrochemical techniques. ... 

Figure 22. The reversible electrochemical and chemical control of the relative positioning of the components of the [2]rotaxane 58-4PF6 in solution.

The formation of the live-coordinate axial complexes ZnTPP(L) has been studied electro-chemically, and thin-layer spectroelectrochemical techniques have been used to demonstrate the electrochemical and spectrochemical reversibility of the process 1184,1185... 

If M is unstable then ipb/fpf will be less than unity. Its magnitude will depend upon the scan rate, the value of the first-order constant k, and the conditions of the experiment. At fast scan rates the ratio ipb/ ip, may approach one if the time gate for the decomposition of M is small compared with the half-life of M-, (In 2jk). As the temperature is lowered, the magnitude of k may be sufficiently decreased for full reversible behaviour to be observed. The decomposition of M- could involve the attack of a solution species upon it, e.g. an electrophile. In such cases, ipb/ipf, will of course be dependent upon the concentration of the particular substrate (under pseudo-first-order conditions, k is kapparent). Quantitative cyclic voltammetric and related techniques allow the evaluation of the rate constants for such electrochemical—chemical, EC, processes. At the limit, the electron-transfer process is completely irreversible if k is sufficiently large with respect to the rate of heterogeneous electron transfer the electrochemical and chemical steps are concerted on the time-scale of the cyclic voltammetric experiment.1-3... 

The auraferraborane clusters of System 7 show a marked dependence on the identity of the MPhj ligand. The As complex is easier to reduce than the corresponding P complex and the reduction product is more stable (tj/j 0.1 s for the As complex, and tj/2 0.01 s for the P complex ). The major difference, however, is found for the oxidations while the P complex shows two well-defined electrochemically and chemically quasi-reversible processes in CV, the As complex only gives one well-defined oxidation peak at a potential ca 0.3 V higher than F ,i for the P complex (i.e. close to F ,2 for the P complex). A difference of ca 0.3 V in the redox potential upon a change from PPhj to AsPhj seems very unlikely based on the other data in Table 18, and the explanation of the observation is probably that a small pre-peak in the CV of the As complex (at a potential close to F ,i for the P complex) is in fact the first oxidation of the As complex. The unexpected small size may be due to a reorganization process prior to electron transfer, which is much slower for the As than for the P complex. 

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