Binuclear complex valency states

The other copper-only binuclear centre to be considered is the CuA or purple copper complex. It is part of the terminal oxidase in mitochondrial respiration, cytochrome c oxidase (COX). Its EPR signature, a seven-line spectrum, has since long been known to be different from the classes type 1 to 3 and arises from two copper ions in a 1.5 valence (or mixed valence) state, first proposed from EPR-analysis of a similar center in nitrous oxide (N20) reductase. There is a close correspondence between the blue and purple states of copper since each of the two copper ions in CuA can be considered as being structurally related to the mononuclear blue site coordination. 

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. 

Optical electron transfer (often designated intervalence transfer, IT, in the case of binuclear mixed-valence transition metal complexes [6, 12]), occurring vertically from the equilibrium configuration of the initial state (Figure 2a). 

It was found that binuclear complexes with delocalized mixed-valence states form an asymmetric 1 3 adduct to 18C6, in which there is a 1 1 stoichiometry at one ruthenium moiety and 1 2 stoichiometry at the other. From the results of this study, it was clear that the valence of the metal center significantly affects the composition of crown ether adducts of metal complexes. 

Photoinduced ET in binuclear complexes with localized electronic states provides at the moment the best test of theory predictions for the solvent dependent ET barrier. This type of reaction is also called metal-metal charge-transfer (MMCT) or intervalence transfer (IT). The application of the theory to IT energies for valence localized biruthenium complexes and the acetylene-bridged biferricenium monocation revealed its superiority to continuum theories. The plots of E vs. E p are less scattered, and the slopes of the best-fit lines are closer to unity. As a major merit, the anomalous behavior of some solvents in the continuum description - in particular HMPA and occasionally water - becomes resolved in terms of the extreme sizes, as they appear at the opposite ends of the solvent diameter scale. 

Mixed-valence complexes are found when the two oxidation states of a binuclear complex are different whereas average-valence (symetrical binuclear) complexes are those giving an infrared absorption intermediate between those of the two monomers with the different oxidation states. 

It is well established that electron localization yields to a polarization of the intramolecular and extramolecular media. Thus, in coordination complexes, a change in oxidation state produces a change in metal ligand bond lengths. Consider for instance a binuclear mixed valence compound where an electron can exchange between two chemically equivalent sites. Since electronic transfer can occur only when the two metal sites have identical environments, a rearrangement of metal ligand bond distances must occur prior to electron transfer. It is then possible to compute in a semi-classical way the contribution... 

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. " ... 

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