Intervalence reactions

Every LVI enters chemical redox interactions in the bulk. According to [19], we define these interactions as intervalence reactions (IVR). 

As far as the intervalent equilibria are concerned, the nonstationary methods are less informative with respect to equilibrium than the stationary methods which we have just considered. Stringent requirements to experimental conditions have to be met in order to obtain the correct data. On the one hand, these requirements are imposed by the conditions which are necessary to achieve the equilibrium for intervalence reactions, and on the other hand, by the necessity to maintain the regime of linear semi-infinite diffusion that only allows for a theoretical analysis. The combinations of the necessary conditions can be rendered by the inequalities ... 

Thus, as we have already found out, the data of chronopotentiometric experiments cannot be presented in simple form such as Eq. (2.9) or (2.10) to calculate the equilibrium stability constants of LVIs by a standard routine. In our opinion, this method is more appropriate for the studies of intervalence reaction kinetics (see the next chapter) rather than equilibrium. However, some information on the intervalence equilibria also can be obtained in certain cases. For example, equilibrium disproportionation constant may be estimated from the shape of the wave, as was pointed out above, ii K > A and the plot in semi-logarithmic coordinates is typically S-shaped as in Fig. 2.10. 

The species of Si(I) form at the end of the non-stationary cathode reduction. They deposit onto the electrode surface resulting in the formation of the film. Elementary silicon can be obtained only at longer times when the electrode film system with appropriate properties is finally settled down and the solid-phase reduction mechanism enters its play (see Chaps. 1 and 5). If, by any chance, the elementary silicon is available in the system, then the intervalence reaction occurs with ions taking an essential part. 

The intervalent reactions proceed reversibly and thus are not shown in the scheme. Unlike for the case of KCl-KF-K2SiF6 molten system, the Si(III) intermediate is least stable in the cryolite-based electrolytes. The Si(II) compounds are deposited onto the cathode surface to give rise to the complications for the industrial process of electrowinning Al-Si alloys [33]. 

The disadvantages of this model are evident—it cannot account for the impact of the intervalent reactions kinetics to the total electrochemical process. That this impact is significant and cannot be ignored was substantiated earlier in the works of Kozin and co-authors summarized in their book [35]. In order to allow for the contribution of the intermediates, the so-called hetero-staged reaction mechanism was proposed, in contrast to Vetter s mechanism defined there as the homo-staged one [34]. 

The first step in this direction has already been made. The results analysed in the last chapters demonstrate the possibility of non-contradictory systematic description of many-electron systems incorporating the effect of intermediate species. However, some problems still remain to be overcome on this way. First of all, the fact of intimate coupling between electrode and intervalent reactions must be deeply understood. 

If the rVRs occur reversibly (that is, the equilibrium relationships of 2.2 or 2.3 type are obeyed), this results immediately in the fulfilment of the Nemst equation for near-electrode space. Then, the irreversibility of an electrode reaction is not detectable in this case. In other words, the kinetics of an electrode reactions cannot show itself when the IVRs are reversible. Here lies the answer on the question why the Vetter s theory on the kinetics of stepwise electrochemical process was developed for the situation of infinitesimal rates (or absence) of coupled intervalent reactions. No chemical kinetics—no electrochemical kinetics ... 

We tried to demonstrate the application of the stepwise mechanism of discharge taking into consideration the intervalent reactions, which accounts for the formation of electrode film systems, which are the active participants of the electrolysis process. 

To begin with, the basic concepts and definitions related to the model of stepwise processes are introduced and rigorously formulated. This model presumes a series of consecutive one-electron electrochemical reactions with conjugated chemical intervalence reactions, every low valence intermediates taking part in these reactions. The properties of a particular system, including the possibility to observe the separate steps, follow logically from the stability of the intermediates that can be characterised by either equilibrium or rate constants of the intervalence reactions. 

Electron transfer reactions within a- and 7t-bridged nitrogen-centered intervalence radical ions, 41, 185... 

Does T differ significantly from unity in typical electron transfer reactions It is difficult to get direct evidence for nuclear tunnelling from rate measurements except at very low temperatures in certain systems. Nuclear tunnelling is a consequence of the quantum nature of oscillators involved in the process. For the corresponding optical transfer, it is easy to see this property when one measures the temperature dependence of the intervalence band profile in a dynamically-trapped mixed-valence system. The second moment of the band,... 

Binary Compounds. The thermodynamics of the formation of HfCl2, of HfCl4, fused sodium and potassium chlorides have been described. The reduction of ZrXj (X = Cl, Br, or I) with metallic Zr or A1 in molten AICI3 has been studied at temperatures from 250 to 360 °C, depending on the halide. The electronic spectra of the initial reaction products were consistent with either a solvated Zr complex or an intervalence Zr "-Zr" species. Further reduction resulted in the precipitation of reduction products which were identified by analysis and i.r., electronic, and X-ray powder diffraction spectra. The stability of the trihalides with respect to disproportionation was observed to increase from chloride to iodide thus ZrC and ZrCl2,0.4AlCl3 were precipitated, whereas only Zrlj was formed. ... 

That the above redox isomer is formed instead of the FenRuin one, can be demonstrated by careful analysis of the MLCT band of the product, as well as by the properties of the intervalence (IV) band. However, it is well known that the [Run(NH >)r,LV,+ ions are generally much more reactive that the [Fen(CN)5L]" analogues toward oxidation by peroxydisulfate (126,128), as required by the lower redox potential at the ruthenium center. A careful mechanistic analysis showed that, although the FeinRun isomer is the thermodynamically stable product, it is not the kinetically accessible one. Then, the reaction evolves as follows ... 

Electron transfer, in thermal and photochemical activation of electron donor-acceptor complexes in organic and organometallic reactions, 29, 185 Electron transfer, long range and orbital interactions, 38, 1 Electron transfer reactions within s- and p-bridged nitrogen-centered intervalence radical ions, 41, 185... 

The charge transfers in polynuclear complexes with metal centres ready to undergo redox reactions are called intervalence charge transfer (IT). These compounds are often characterized by low-energy M-M transitions and thereby have intense colour (eg Prussian blue and its analogues). 

Figure 1. Potential energy as a function of reaction coordinate for a self-exchange reaction. AE, energy barrier for thermal electron transfer (weak coupling) AE2, energy of an intervalence transition which is possible for the system.

The results in Table III show that there is a rough parallel between the rates of net electron transfer from Ru(II) to Co (III) and the intensity of the intervalence band observed for the Ru(II)-Ru(III) mixed valence complex. Any such parallelism—more exactly a parallelism between AS+ and the intensities—would imply that a nonadiabatic factor does affect the rates of electron transfer. Unfortunately, it is not possible to examine this relationship in the sensitive region where the reactions are strongly nonadiabatic. The nir bands are very broad, and is difficult to measure extinction coefficients that are less than about 5 M"1 cm-1. 

There are three principal modes of ET, namely, thermal, optical and photoinduced ET, and these are shown schematically in Fig. 1. Optical ET differs from photoinduced ET in that ET in the former process results from direct electronic excitation into a charge transfer (CT) or intervalence band, whereas photoinduced ET takes place from an initially prepared locally excited state of either the donor or acceptor groups. Photoinduced ET is an extremely important process and it is widely studied because it provides a mechanism for converting photonic energy into useful electrical potential which may then be exploited in a number of ways. The most famous biological photoinduced ET reaction is, of course, that which drives... 

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