Mixed valence compounds electronic spectra

The subsequent reactions differ in the cases of the reduced and mixed valence enzyme. The reaction of the mixed valence compound may well be expected to be simpler than that of the fully reduced enzyme, as electron transfer from cytochrome a and Cua cannot be involved. As the spectrum due to compound A from the mixed valence enzyme disappears, so a new band forms with maximum intensity at 605-610 nm. This is compound C, which appears to involve peroxo-bridged Fe" and Cu . At one time it was thought that cytochrome in compound C was Fe", but this view seems unlikely. Another possibility involves peroxide bridging Fe" and Cu. These suggestions have all been assessed in the light of UV-vis absorption and ESR spectroscopy. They are shown in Figure 59. 

Considerable work has gone into the synthesis and reaction chonistiy oi bifenocenes. The X-ray structure of 3,r"-(a-keto-propane-l,3-diyl)-l,r bifeiTocene has been derermined and the Fe Mbssbauer spectrum and EPR results on the related mixed valence compound 3,r"-(propan-l,3-diyl)-l,r-biferrocenium triiode suggests a localized electronic structuie. Further work on the electron transfer in related compounds has been done and the effect of bending modes on the intramolecular electron transfer rates in tetraethylbifertocinium triiodes has been determined in addition to the effect of substituents. ... 

Other compounds with the lantern structure include the acetamidates Rh2(MeCONH)4L2 and the mixed-valence anilinopyridinate Rh2(ap)4Cl (Figure 2.39), which has an unusual ESR spectrum in that the electron is localized on one rhodium [79]. 

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... 

In recent years several complexes have been prepared that contain oxidant and reductant ions of the same element, with the same inner-sphere ligand environment and a symmetrical bridging group (Table VI). In the most-studied cases, analogous compounds with one more and one less electron are known, forming a redox series A + XA+(env), A+ X A(env), A X A(env), and the electronic spectrum of the mixed-valence species contains a unique band in addition to the bands characteristic of the separate A+ X and A X chromophores. 

The reaction between 1 equiv. of the sterically bulky COT" (GOT" = l,4-bis(trimethylsilyl)cyclooctatetraene) ligand and TiCl3(THF)3 did not result in the clean formation of an analogous compound to 160.119 Instead, a mixture of the Ti(n) species (COT")2Ti, the mixed valence Ti(m)/Ti(iv) species (COT")Ti 2(/x-Gl)3 166 and the asymmetric Ti(m) dimeric species (GOT")Ti(/x-Gl) 2(THF) 167 were formed in yields of 15%, 30% and 50%, respectively. X-ray crystallography confirmed that only one of the Ti centers in 167 coordinates to a molecule of THF, although this THF was released when 167 was warmed to 80 °G in vacuo. The EPR spectrum of 167 confirmed that the two Ti centers contained unpaired electrons while that of 166 was consistent with a GOT" Ti(m) species. No unusual line broadening was observed in the EPR spectra of 166 when it was cooled this suggested that the EPR relaxation time was relatively fast compared to any electron transfer between the two titanium centers. 

TTF)5Pt(CN)4.2 CH3CN (1) In this compound (Fig. la), the unit cell contains two independent Pt(CN)4(TTF)2 5 blocks [6]. The TTF molecules of each independent unit form centrosymmetric isolated pentamers. The Pt(CN)4 " dianions are separated by the disordered acetonitrile molecules. The structural features of the TTF molecules show that the central molecule of the pentamer is neutral, while the other four are fully oxidized, indicating mixed valence in the isolated pentamers. This new pq>e of organization has particular optical properties. The room-temperature electronic absorption spectrum (Fig. 2) e ibits the two... 

Ionic and covalent materials can combine in any group of valence compounds forming a class centered on simple or complex phases with four electrons per atom. Valence compounds with three and five electrons per atom are the nearest nel bors. In each of these subgroups we find that an increase in the atomic weight tends to increase the metallic interaction between the atoms and to alter the structure, However, most of the valence compounds are substances with mixed (ionic-covalent) bonds and with a gap in the electron energy spectrum, i.e., they are semiconductors. 

During the last 5 to 10 years there has been much interest in electron spectroscopies (Hiifner and Steiner 1982), since it was realized that these also suggest a very different picture of mixed valence Ce compounds. Valence photoemission spectroscopy (PES) studies showed that the f-spectrum of Ce has weight at (8p ) — 2eV below Ep (Platau and Karlsson 1978, Johansson et al. 1978). It was further found that core level X-ray photoemission spectroscopy (XPS) measurements were hard to understand unless A (Fuggle et al. 1980b) is much larger than previously assumed. This discrepancy between the interpretations of spectroscopic and thermodynamic data showed the need for a theoretical analysis, based on a microscopic model, of what kind of information can be extracted from different experiments. This was further emphasized when PES studies showed f-character in the spectrum both at —2eV and close to 8p = 0 (Martensson et al. 1982). This observation created a lively debate about how to interpret the PES spectra. 

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