Mixed-valence compounds absorption

Mixed-valence chemistry was reviewed in the late 1960 s both by Robin and Day (4) and by Hush (5). Their work provided the beginnings of a theoretical background for understanding the properties of mixed-valence compounds including the low energy absorption bands which have been termed Intervalence Transfer (IT) or Metal-Metal Charge Transfer (MMCT) bands. 

Other Papers.—Various iron species prepared by the vacuum pyrrolysis of acetyl-ferrocene-furfural resins at 400°C have been studied by Mossbauer spectroscopy. These consist of an amorphous glass-like carbon matrix containing free iron atoms, Fe+ ions, iron clusters, superparamagnetic iron, and ferromagnetic iron.333 The effect of pressure of up to 50kbar on the absorption spectra of five iron(m), two iron(n) and one mixed valence compound has been studied. In six of the compounds, but not in basic ferric acetate or soluble Prussian Blue, the observed pressure-induced bands were assigned to d-d transitions of converted iron(n) for the ferric compounds and to spin-forbidden d-d bands for the ferrous compounds. The charge-transfer band from iron(n) to iron(m) in soluble Prussian Blue showed a blue shift at pressures up to 7.2 kbar.334... 

Vivianite, Fe2+3(P04)2.8H20, is the classic example of a mineral showing an intervalence charge transfer transition (Wherry, 1918 Bums, 1981). Vivianite has a diagnostic indigo-blue colour and a well characterized Fe2+ —> Fe3+ IVCT absorption band in the polarized spectra illustrated in fig. 4.12 and is the datum with which electron interaction parameters for other minerals are compared. The chemical formula of vivianite is not indicative of a mixed-valence compound. However, the pale-green colour of newly cleaved vivianite crystals or fleshly... 

The blue precipitates obtained on mixing Fe3+ with [Fen(CN)6]4 or Fe2+ and [Fem(CN)6]3 have long been known both products are Fe4n[Fen(CN)6]3T5H20. This compound presents an intense absorption band at 700 nm due to transition from the ground state to an excited state in which an electron is transferred from an Fe11 to an Fem site. Prussian blue may be considered as the archetype of mixed valence compounds it contains two identical metals in different oxidation states. These compounds have played, and continue to play, a crucial role in the study of electron transfer phenomena.51... 

Metal-metal charge transfer or intervalence transitions occur in many mixed-valence compounds and may involve cations of the same metal (e.g., Fe — Fe, TP+ — TP ) or different metals (e.g., Fe TP ). The majority of intervalence transitions take place between octahedrally coordinated cations where the octahedra are edge-sharing the transitions are strongly polarization dependent and occur only when the electric vector is oriented along the appropriate metal-metal axis in the crystal structure. The absorption peaks are often located in the visible region. 

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. 

For 82 complexes of type III, the corresponding absorption band is expected to occur at higher energy because both 7t orbitals of the ligand interact strongly with the metal atoms. Particularly interesting is the very intense band at 14.2 kK in [(NH3)5Ru(82)Ru(NH3)5]Cl4. A comparable band is not found in the related compounds of type Ila structure [e.g., [(CN)5Co(82)Co(CN)5] and aqueous Cr(82)Cr ]. It has been proposed that the central unit in [(NH3)5Ru(82)Ru(NH3)s] is best formulated as Ru —(82) —Ru" (25, 44). 8uch a mixed-valence complex could exhibit the intense band observed 106). 

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