Mixed-valence compounds electronic coupling

Based on the strength of the electronic coupling between the metals, Robin and Day [105] have developed a system in which mixed-valence compounds are broadly distinguished in three classes. In a very weakly coupled or Class I material, only the properties of the individual mononuclear species are observed due to the lack of communication ( = 0). In the other extreme, a Class III com-... 

The vast majority of the coordination compounds of Os that have been prepared are in the oxidation states 11 and III. Moreover, many of these compounds show reversible or well defined Os / couples in which the electronic and redox properties at the metal are controlled by the a-donor, 7r-acceptor, and r-donor properties of the ligands. Indeed, the study of the redox behavior in Os / and Ru / species, metal ions in which octahedral coordination is almost universally retained in both redox partners, has been central in recent developments to parameterize metal centered redox processes as a function of ligand donor and acceptor capacity. The chemistries of Os and Os are, therefore, intimately linked, and have been extended to studies of important mixed valence Os / binuclear and polynuclear species (see Mixed Valence Compounds). For the purposes of brevity and convenience, this section will deal with Os and Os complexes together. The extensive literature on Os / complexes has been developed with a very wide range of donor ligands a comprehensive assessment of this work is beyond the scope of this article, and the reader is directed to published comprehensive reviews. " ... 

A number of effects or parameters vary with distance according to exponential, or almost exponential, laws. This is of course the case for the basic f(,b parameter, and this occurs also for several related experimental observables, such as electron and energy transfer rate constants, and also magnetic J couplings. This general behavior is ultimately linked to orbital overlap considerations, and is discussed in more detail below for the case of mixed-valence compounds. 

With strong metal-metal interactions across a bridging ligand, the valence redox orbitals are delocalized molecular orbitals both metal and ligand in character. In mixed-valence compounds, different, discrete oxidation states do not exist since the site of oxidation is delocalized. Strongly coupled systems are like metal-metal bonds in that their electronic and chemical properties are significantly modified from those of related monomeric complexes. As with metal-metal bonds, such compounds can have an extensive multiple oxidation state chemistry based on delocalized molecular orbitals. 

Theories for electron-phonon effects in mixed-valence compounds, which take into account a breathing mode coupling of nearest neighbors to the rare-earth ion, have been developed (Sherrington and Molnar 1975, Hewson 1979). They were used to explain phonon spectra in systems like SmQ 75Y0 25S (Mook et al. 1978). 

Dramatic examples of interacting electronic states include intervalence bands in mixed-valence compounds and many charge-transfer bands where coupled states influence the distribution of electron density, as well as the spin-crossover spectra. 

In many ways the electronic coupling in both the neutral and the mixed-valence compounds can be reduced to the simple orbital diagram depicted in Fig. 7 [26]. In the neutral complex only the MLCT transition is observed, but for the singly oxidized radical cations three electronic transitions are possible the LMCT, the IVCT (in a fully delocalized compound this is better described as charge resonance transition), and an LMCT. For carboxylate linkers the LMCT is not observed because the CO2 Tt orbitals are too low in energy, but substitution of O by S or NR raises the energy of this filled n orbital. 

Fig. 2. Potential energy curves for mixed-valence compounds with negligible (a), weak (b), and strong (c) electronic coupling. In (b) and (c) the dashed curves represent zero-order states.

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... 

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