Mixed-valence electronic coupling

Fig. 3.3 Definition of the electrolyte window Eg for (a) liquid and (b) solid electrolytes. The EpA and/or Epc of solid electrolytes may lie in either a band of one-electron stales as in (a) ot a multielectron redox couple as in (b) for liquid electrodes, they lie in a mixed-valence redox couple...

Fig. 8. Scheme of the electronic structure of (A) [3Fe-4S] centers and (B) [4Fe-centers according to the standard model. The thin and thick dashed fines indicate the Emtiferromagnetic and double exchEmge coupling, respectively. Configurations a and b correspond to the two possible locations of the excess electron in the mixed-valence pair. In part (B), the local spin values are Sc = Sd = 2 in the case of [4Fe-4S] centers and Sc = Sd = i in the case of [4Fe-4S] + centers. 

A.464 A purple-black, mixed-valence Ir11 Ir1 binuclear compound, [(Ir(cod)(/u-L) 2]BF4 (L = pz, 4-Mepz), is synthesized from the reaction of [Ir(cod)(//-L)]2 with NOBF4. The binuclear cationic radical exhibits an EPR spectrum showing hyperflne coupling to two equivalent Ir. Cyclic voltammetry studies have shown a reversible, one-electron oxidation.4... 

These results suggest that the critical factor in the substrate-mediated intermolecular interactions which occur within the close-packed DHT layer is the inherent strong reactivity of the diphenolic moiety with the Pt surface. The interaction of adsorbates with each other through the mediation of the substrate is of fundamental importance in surface science. The theoretical treatment, however, involves complicated many-body potentials which are presently not well-understood (2.). It is instructive to view the present case of Pt-substrate-mediated DHT-DHT interactions in terms of mixed-valence metal complexes (2A) For example, in the binuclear mixed-valence complex, (NH3)5RU(11)-bpy-Ru(111) (NH 3)5 (where bpy is 4,4 -bipyridine), the two metal centers are still able to interact with each other via the delocalized electrons within the bpy ligand. The interaction between the Ru(II) and Ru(III) ions in this mixed-valence complex is therefore ligand-mediated. The Ru(II)-Ru(III) coupling can be written schematically as ... 

A theoretical formalism is available for understanding optical charge transfer processes in a variety of chemical systems (mixed-valence ions, donor-acceptor complexes, metal-ligand charge transfer chromophores, etc) where the extent of charge transfer is large and where electronic coupling between the electron donor and acceptor sites is relatively small. 

A vibronic coupling model for mixed-valence systems has been developed over the last few years (1-5). The model, which is exactly soluble, has been used to calculate intervalence band contours (1, 3, 4, 5), electron transfer rates (4, 5, 6) and Raman spectra (5, 7, 8), and the relation of the model to earlier theoretical work has been discussed in detail (3-5). As formulated to date, the model is "one dimensional (or one-mode). That is, effectively only a single vibrational coordinate is used in discussing the complete ground vibronic manifold of the system. This is a severe limitation which, among other things, prevents an explicit treatment of solvent effects which are... 

There are at least two ways in which detailed information about electron-vibrational coupling strengths can be obtained for mixed-valence complexes. Both are based on the fact that such coupling will be reflected in modifications of the vibrational spectrum. Thus, for example, coupling to antisymmetric modes in a symmetric ion will modify intensities and frequencies of the modes involved. 

The bridged ion-radicals are usually subdivided into three classes (Robin and Day 1967, see also Dumur et al. 2004). Class I—the redox centers are completely localized and behave as separated entities, class II—intermediate coupling between the mixed-valence centers exists class III— the redox centers are completely equivalent being enriched with an unpaired electron by the half-to-half manner. For instance, the cation-radical of A,A,A, A -tetraphenyl-p-phenylenediamine belongs to class III (Szeghalmi et al. 2004). 

In summary, by combining the information obtained from the new mixed-metal clusters with the progress made in describing the spin coupling of delocalized mixed valence dimm with double exchange, we can anticipate new insight into the electronic structure of many clusters of interest in bio-inorganic chemistry. 

Three oxidations states are potentially available in a binuclear iron center. Enzymes with octahedral fi-o o bridged iron clusters can be isolated in each of the three states the diferric and diferrous states appear to be the functional terminal oxidation states for most of the enzymes, while the mixed valence state may be an important intermediate or transition state for some reactions (Que and True, 1991). In these enzymes the cluster participates primarily as a two-electron partner in the redox of substrates, perhaps using sequential one-electron steps. Without additional coupled redox steps the enzyme is in a new oxidation state after one turnover. In contrast only the diferric and mixed valence oxidation states have been found for 2Fe 2S clusters. The diferrous state may not be obtainable because of the high negative charge on [2Fe 2S(4RS)] versus -1 or 0 net charge for the diferrous octahedral (i.e., non-Fe S) clusters. The 2Fe 2S proteins either are one-electron donor/acceptors or serve as transient electron transfer intermediates. 

The term mixed valence state has been used in two connotations (1) to mean two coupled irons with different formal charge (e.g., Fe Fe ) and (2) to mean two (or more) irons coupled and electronically delocalized to have fractional formal charge (e.g., 2Fe from delocalized Fe -t- Fe " "). We use mixed valence only in the former meaning. 

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