Mixed-valence compounds intervalence transitions

The reduction of molybdate salts in acidic solutions leads to the formation of the molybdenum blues (9). Reductants include dithionite, staimous ion, hydrazine, and ascorbate. The molybdenum blues are mixed-valence compounds where the blue color presumably arises from the intervalence Mo(V) — Mo(VI) electronic transition. These can be viewed as intermediate members of the class of mixed oxy hydroxides the end members of which are Mo(VI)02 and Mo(V)0(OH)2 [27845-91-6]. MoO and Mo(VI) solutions have been used as effective detectors of reductants because formation of the blue color can be monitored spectrophotometrically. The nonprotonic oxides of average oxidation state between V and VI are the molybdenum bronzes, known for their metallic luster and used in the formulation of bronze paints (see Paint). 

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... 

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. 

Another special spacer consists of porphyrin (either metallated or not) substituted in the 5,15 positions (see Figure 12b). Intervalence transitions have been observed when the ferrocene groups are electrochemically oxidized [36]. The metal-metal distance is >10 A. No estimation of the l b coupling was reported, but the mixed-valence compounds would correspond to class III, or II/IIl. 

TABLE 2. Characteristics of the intervalence transition of the mixed-valence compounds. Solvent acetonitrile. 

Figure 1.46). ° The monocation 97 can be considered as a push-pull chromophore and exhibits a / i.o6 (HRS) value twice that of its neutral precursor 97 and the fully oxidised compound 97. Complex 97 is a class II mixed-valence complex, retaining charge localisation, but with enough electronic coupling between the redox centres to permit intervalence charge transfer transitions. As the redox couples 97/97" and... 

The intervalence transition is often in the visible region and since the transition is allowed, a mixed valence complex often has a very strong color. A good example is Prussian blue, which can be made by adding Fe + to K4Fe(CN)( , where the valence state of Fe is +2. Alternatively, Fe + may be added to KjFelCN), where the valence state of Fe is -1-3. If compounds with the same valence are mixed, there is no strong color. In the present case, the system is not strictly symmetric since one Fe is connected to C while the other is connected to N in CN". The lack of symmetry guarantees a unique product ( TumbuH s blue is the same as Prussian blue). 

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