Mixed-valence complexes systems

The low effectiveness of the [Rh2"( Jt-OAc)4] system [31] in the oxygenation of alkenes has been attributed to its high oxidation potential to form the mixed-valence complex [Rh2" " ( Ji-OAc)4] on the basis of a relationship between the abihty of the complexes to transfer one electron and the effectiveness of the catalysts in allyhc oxidation, suggested by Kochi [33]. 

The formation of the unsaturated intermediate Run(OEP) from the hydrido-complex RumH(OEP) is supposed to occur from a LMCT excited state. As a consequence, the dimeric [RuH(OEP)]2 with a metal-metal interaction is formed [245]. Irradiation of some systems containing Fein(Por)N3 leads to p-nitrido bridged binuclear mixed-valence complexes [(Por)Fem-N-Fe,v(Por)] [134, 162], In both cases photochemistry was used as a conventional preparative route for synthesis of the binuclear complexes. 

There are several photocatalysts mimicking hydrogenase activity that are not based on metalloporphyrin systems. Among them there are mixed-valence complexes of rhodium or iridium, [41] as well as complex systems encompassing photosensitizers (eg ruthenium complexes) attached to a catalytic bimetallic centre [43], The design of more sophisticated systems approaches that of photosynthetic processes [44],... 

An important application of the work on mixed valence complexes in which the metal ions are weakly but not too weakly coupled is in resolving the Franck-Condon barrier to electron transfer into the separate contributions from the inner-sphere and the solvent. Some work directed to this goal has been reported (62, 63). Because of the nature of the dependence of the solvent barrier on distance, those systems in which the separation of the metal centers is large will be particularly instructive. When this separation is sufficiently large, it may be possible to estimate the radius of the dielectrically saturated region about each ion so as to compare it with the radius of the coordination sphere itself. 

The intellectual push to study mixed-valence complexes was provided by the publication in 1967 of two review articles, by Allen and Hush (2) and Robin and Day (3), on the physical properties of mixed-valence systems. These were followed by Hush s publication (4) of his theoretical model of intervalence transitions, which provided a link between the properties of mixed-valence complexes in solution and the Marcus theory of intermolecular electron transfer (5, 6). The review by Robin and Day classified mixed-valence complexes into three types class I,... 

Fig. 1. Potential energy-configuration diagram of initial and final states for (a) symmetric mixed valence complex and (b) asymmetric mixed valence system.

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). 

Table I compiles the electrochemical and metal-metal absorption band spectral data of novel mixed-valence complexes (35 46) that are good candidates for valence delocalized systems. The decision as to where to draw the line between class II and III mixed-valence complexes is a difficult one to make. The Creutz-Taube ion has undergone an extremely rigorous physical characterization by just about every method known (see below), and it is only recently that the preponderance of evidence strongly favors a class III description. The same degree of examination should be applied to the complexes of Table I to fully justify their class III assignment.

Mixed-valence complexes exhibit interesting properties when significant electron coupling occms between the donor and acceptor sites. Most of the systems are Ru(II)/Ru(III) species, but if electronic coupling is sufficient, there is a... 

This is the most direct experimental manifestation of the existence of an electronic interaction. It can occur spontaneously in mixed-valence complexes, but also in bimetallic systems after a photochemical excitation (photoinduced electron transfer). The general theory considers electron transfer as a special case of radiationless transition, with a perturbative treatment based on Fermi s Golden Rule [42]. In the nonadiabatic case, the rate constant can be written as [43] ... 

Mixed-valence complexes based on this end group were first developed by Felix and Ludi [70], and their basic properties are reviewed elsewhere [1]. The longest system is based on the bis(pyridyl)ethylene ligand, and the metal-metal separation (14 A) is... 

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