Mixed-valence complexes coupling model

From Eq. (19), Eop should be linearly proportional to the solvent term (1 IDop - 1 IDa). This relationship has been demonstrated for weakly coupled mixed valence complexes (see Section (II.C.2.c) and this gives qualitative support to the Hush model. 

Piepho has responded to the criticisms of the PKS model by developing an improved version, the MO vibronic coupling model for mixed-valence complexes (32). Multicenter vibrations are now considered and a molecular orbital basis set (as with the three-site model) is used. This model was used to calculate band shape and g values for the Cretuz-Taube ion (33). The MO vibronic coupling model is admittedly more empirical than the three-site model but it has the advantage in being applicable to all mixed-valence complexes. 

In a recent study, Curtis and coworkers (34) developed an analysis of electronic coupling in mixed-valence complexes that combines electrochemical results with an extension of Mulliken s theory of donor-acceptor interactions. The approach was applied to class II and near class III complexes. It was found that the degree of electronic coupling in these systems is at least three times that which would be predicted based solely on spectroscopic measurements (Eq. 15 of the Hush model). However, the electronic coupling determined for complexes that were very close to, if not of, class III character was significantly smaller than that predicted by Eq. (16). 

An extended MuUiken-Hush treatment of mixed-valence complexes can be found in Chapter 2.45. In other studies, the effect of bridging ligand topology on metal-metal coupling was analyzed in terms of quantum interference. Ferretti et al have proposed a model for the study of the optical properties of mixed-valence complexes, and Coronado et have developed a general... 

The book Electrons in Chemical Reactions by L. Salem places electron transfer in a broad context ranging from basic wave mechanics to ion-solvent interactions. Sutin has reviewed theories of electron transfer with emphasis on nuclear, electronic, and frequency factors and Wong and Schatz have given a useful unified account of the dynamics of mixed-valence complexes using the vibronic coupling model. ... 

The encounter complexes exhibit high degrees of charge-transfer [20, 91], and on the basis of absorption and emission data electronic coupling matrix elements for similar complexes (exciplexes) have been determined [205] which are comparable to those of mixed-valence metal complexes commonly used as prototypical models for the bridged-activated complex in inner-sphere electron transfers [2, 26, 197]. Accordingly, we ascribe the unusually high rate constants, their temperature-independence, and their on-Marcus behavior to an inner-sphere electron transfer process [31]. 

There exist a vast number of discrete, polymetallic coordination clusters both of the metal-metal bonded type and linked by a tremendous variety of bridging ligands, notably carboxylates. Such compounds are not coordination polymers but oligomers and hence can be more soluble, more well-defined and easier to characterise than coordination polymers, for which they can serve as useful model systems. We will discuss discrete, self-assembled complexes of semi-protected metal ions that act as hosts in solution or as 3D capsules in the next chapter and we will not cover these systems in detail here except to note in passing a couple of examples that are of particular interest. One particularly prominent cluster is Muij-acetate, the mixed-valence compound... 

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