Mixed-valence intermediates

Hydroxide. Freshly precipitated cerous hydroxide [15785-09-8] Ce(OH)2, is readily oxidized by air or oxygenated water, through poorly defined violet-tinged mixed valence intermediates, to the tetravalent buff colored ceric hydroxide [12014-56-17, Ce(OH)4. The precipitate, which can prove difficult to filter, is amorphous and on drying converts to hydrated ceric oxide, Ce02 2H20. This commercial material, cerium hydrate [23322-64-7] behaves essentially as a reactive cerium oxide. 

Such di- and tetranuclear complexes can have an extensive redox chemistry. For example, the Ptn compound [Pt2Cl(NH3)3(thym)2]+ (thym = methylthymine anion) is oxidized to Ptin via intensely colored mixed-valence intermediates.51... 

The separation between the two reductive steps in either pathway can not be more than 0.1 v because [TPPFe(II)] is observed to reduce at Ej/2 = — 1.03 v. The fact that the mixed valence intermediate [TPPFe(III)-0-Fe(n)TPP]- does exist is not proven electrochemically but by EPR and visible spectral observation after partial reduction at — 0.93 v. A paramagnetic species with a single asymmetric EPR peak at g = 1.95 is produced and stable only at 77 K. The same species can be produced through the reduction of (TPPFe(HI))20 by [TPPFe(II)] (produced electrochemically in situ at — 1.1 v). Thus the reduction of (TPPFe(III))20 can proceed in two facile one electron steps which have very similar reduction potentials. 

Mixed- Valence Intermediates as Ideal Targets for Spectroelectrochemistry (SEC)... 

In addition, mixed-valence intermediates feature prominently in mechanistic approaches aimed at electron-transfer processes, including ultrafast valence exchange and multielectron catalysis (where mixed-valency during stepwise processes is inevitable) The modelling and the classification derived have... 

To detect mixed-valence intermediates via SEC in solution requires the presence of this intermediate in sufficient concentration. It is thus not necessary to have a nearly 100% participation of the inteimediate in an equilibrium as quantified by comproportionation constants K > 10. The problems associated with small Ac values of mixed-valence intermediates have been extensively discussed by Richardson and Taube, and that equilibrium parameter can be considerably enhanced by changing the environment such as the temperature, the solvent (complex 3 ) or the electrolyte. This observation demonstrates the sensitivity of mixed-valence intermediates with respect to the medium. 

The formation of mixed-valence intermediates is not limited to purely electron-transfer redox series, as pointed out before and as described below however, the overall reversibility is typically checked by regenerating the starting spectrum to 100% after completing the backreaction. If the method is selective only for the intermediate state, as, e.g., for EPR active odd-electron species in equilibrium with EPR-silent states, or if the two-step redox system can be analysed via stepwise monitoring, " the spectral features of an intermediate as distinct from those of the neighbouring redox states can be well identified. A case in point is discussed further below by example of intermediates [(C R )M(p-L)M(C R )]". ... 

An important aspect in applying SEC methods to the study of mixed-valence intermediates concerns the timescale."" The frequently asked question as to the... 

Typical responses of redox systems with a mixed-valence intermediate are shown in Figures 3.1 and 3.2, illustrating the rise (and decrease) of the IVCT absorption band as an often separated additional feature on (stepwise) electron transfer. The spectral behaviour as in Figure 3.1 for the Fe "Fe" (or, better. 

For comparison, the essential parameters of the near-infrared band for the pyrazine-bridged mixed-valence intermediates 1-4 are summarised in Table 3.1. 

For instance, trinuclear species such as (p3-L)[Ru(acac)2]3 can be oxidised in three one-electron steps of which the first two produce mixed-valence intermediates (Ru Ru°Ru and Ru Ru Ru"). For some ligands L, such as dqp, the EPR spectroscopically characterised first (i.e. odd-electron) intermediates with metal-centred spin do not exhibit a detectable band in the near- or midinfrared region." The reason for this behaviour is not yet clear, especially since related... 

Summarising, spectroelectrochemical techniques are ideal for studying mixed-valence intermediates ... 

The heme/Cu potential depends both on the site s protonation state and on the redox states of heme a and Cu cited is the best guess potential in the mixed-valence intermediate ready to bind O2. 

Type III copper sites are considered particularly interesting because they are EPR-silent, in spite of having two copper(II) ions. This dicopper(II) centre may be synchronously reduced to a dicopper(I) one by accepting two electrons at one step. A mixed-valence intermediate cannot, however, be ruled out. 

Valence instability is characterized by a strong coupling between the f shells and delocalized itinerant electrons. Mixed valencies, intermediate valencies, valence fluctuations and heavy-fermion systems are the consequences of such couplings. An overview is given by Flouquet et al. (1982). We shall discuss those groups of compounds in which such phenomena are found. 

The stoichiometric oxides of the rare earths are a light pastel color except for what is Pr02 and Tb02 from gravimetric measurements which are black and red respectively. The mixed valence intermediate oxides including the higher oxides of Ce, Pr and Tb and the substoichiometric sesquioxides are all deep colors. 

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