Mixed-valent dimer

With regard to the latter point, the absence of a mixed valence transition in the oxidized low coverage polymer case is an important point. No mixed valence transtiion was observed over the whole range of oxidation (0-100%) studied. This indicates that whereas the D22+ aggregate is stable under these conditions, the mixed valent dimer analog, D2+, is not. At least in these polymeric matrices, therefore, the stoichiometric requirement for observation of the mixed valence state appears to involve (D2+)n where n > 2. 

Spectroscopic and EPR data showed that these binuclear semiquinone complexes are situated at the borderline between anion radical complexes and metal-centered mixed-valent dimers. 

Zhang et have published a quantum-mechanical treatment of absorption line shapes in bridged, mixed valent dimers. This treatment allows vibronic perturbations to mix the electronic potential-energy surfaces in complexes in which the exchanging electron is completely or partially delocalized. When this three-center treatment is applied to the Creutz-Taube ion, it is consistent with complete delocalization of the odd electron and with very little mixing of the three-center electronic states in this complex. 

High-valent iron also occurs in -nitrido bridged dimers with linear [Fe °-N=Fe" ]" and [Fe =N=Fe ] " cores [209, 210] (and references therein). Such compounds have been prepared first by thermolysis [247] or photolysis [248] of iron(III)-porphyrin complexes with an azide ligand, (N3). Mixed-valent iron-nitrido porphyrin dimers exhibit valence delocalization as can be inferred from the... 

Figure 4 Crystal structure of the mixed valent oxide Te2Os. The [Te(VI)Og] octahedra are linked to chains via common edges, and the chains are connected by [Te(IV)04]2 dimers.

A second pathway involves the formation of a mixed-valent [Fe2+, Fe3+]-superoxo species that could react with a second molecule of 36 to form a peroxo-bridged tetranuclear [Fe +, Fe2+] cluster. Homolytic cleavage of the peroxo 0-0 bond followed by electron transfer and rearrangement would yield 37. These two pathways differ in their oxygen stoichiometry pathway 1 has a ratio of 02/reduced iron dimer of 1 1, whereas pathway 2 has a ratio of 1 2. Manometric measurements of 02... 

A series of hemocyanin and tyrosinase active site derivatives (Fig. 23) can be prepared61"66), allowing systematic variation of the binuclear copper active site and chemical perturbation for spectral studies. In the simplest derivative, met-apo, one copper has been removed and the remaining copper oxidized to the spectroscopically accessible Cu(II). Next in complexity is a mixed-valent binuclear copper site. The Cu(II), in this half-met derivative, exhibits open-shell d9 spectroscopic features and the Cu(I), though spectroscopically inaccessible, can still be studied by comparison to the met-apo derivative. Two derivatives have formally binuclear cupric sites met, which is EPR-non-detect-able, and dimer, which exhibits an intense broad EPR signal. Spectroscopic study of these derivatives has led to the present picture of the coupled binuclear copper protein active site shown at the bottom of Fig. 23. 

Dimers of M centers occur in the mixed valent compounds GayXs (X = Br, I) = (Ga" )2 [GayXe] " which contain ethane-type [GayXe] " units with Ga-Ga single bonds. As in the dihalides, mixtures of complex and simple ions combine also in InsXy = (In+)3[In2Br6]Br, which contains the ethane-like units with M-M single bonds as weU. ... 

Oxidation of the [CpFe(CO)2]2 dimer leads to a cation which retains one or more bridging carbonyl groups, but in the presence of coordinating ligands this dissociates into the mononuclear 17e CpFe(CO)2 radical and CpFe(CO)2L+.150 For details on mixed-valent carbonyl dimers, please refer to the following section. 

The electrochemistry of dinuclear vanadium o-A-salicylideneamino-ethylphenyl complexes were investigated.372 Conversion of Viv—Viv dimers to Vv—Vv dimers was reported and serves as a representative model system for oxidation reactions.294 296 A related class of these dinuclear complexes form mixed valence vanadium(IV/V) complexes upon oxidation and exhibit an unusual electron delocalization over the V203 3+ core.310 A V111—Vlv mixed valent, dinuclear bis(salicy-lidene)ethylenediamine (salen) complex underwent multielectron oxidation with dioxygen to yield a Vv—Viv mixed valence complex.302... 

The trinuclear structure of 40 was elucidated by X-ray crystallography. The molecule lies on a twofold axis through Zr(2), and the Zr-Zr and Zr-H distances are represented schematically in Figure 6. The three //-hydride ligands and three zirconium atoms are essentially coplanar, with //-hydrides above and below the plane. There are five hydrides and three zirconium atoms so compound 40 is formally mixed-valent Zr(m)Zr(lv)2. The shorter Zr-Zr distances in 40 compared to those seen in Zr(iv) dihydride dimers are consistent with the presence of Zr-Zr bonding.24... 

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