Intervalence compounds

An increase in the electronic coupling interaction results in the disappearance of the ET barrier and complete delocalization of the transferred electron between the donor and the acceptor. Such effects have been extensively studied for intramolecular ET in bridged intervalence compounds [57]. As regards intermolecular systems, the only spectrally and structurally characterized system has been that of NO+/arene complexes [28]. 

Hush-type intervalence band occur in the IR region. From the op = H b assumption for Class-Ill intervalence compounds in solution, consistent analyses of Class-II and Class-Ill compounds for both metal and organic-centered intervalence complexes are obtained in several instances [145]. 

Scheme 30. Saturated-bridged bis(diazenium) and bis(hydrazine) intervalence compounds.

Not all mixed valence compounds show intervalence bands. Those that show not the slightest evidence for them are often called class 1. In class 2 intervalence compounds the intervalence-transfer band may well dominate the visible spectrum, swamping the spectra of the individual ions. However, these may be seen the important point is that the different ions retain their chemical individuality. Soluble Prussian blue is a case in point. This contains Fe and Fe bridged by CN ligands. Isotopic tracer measurements show that there is no doubt—the Fe" is bonded to the N atom of the CN ligand and the Fe to the C. If radioactive Fe is used in the preparation and the compound subsequently decomposed, the activity remains in the iron of the same valence state in which it was incorporated. A class 3 also exists, exemplified by the IJ anion, in which it is not possible to associate a unique valence state to individual metal ions typically, but as our example shows, not always, they are structurally indistinguishable. When they form an... 

Tominaga K, Kliner D A V, Johnson A E, Levinger N E and Barbara P F 1993 Femtosecond experiments and absolute rate calculations on intervalence electron transfer of mixed-valence compounds J. Chem. Rhys. 98 1228—43... 

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). 

The localized-valence halogen-bridged platinum(II)/platinum(IV) complexes (175) are of interest because of their highly anisotropic electrical and optical properties. The complexes are characterized by broad and intense intervalence bands in their electronic spectra.542 Partial oxidation of [PtCl(dien)]Cl with iodine yields the mixed valence species [PtI(dien)][PtI3(dien)]I2,543 but when 4Me-dien is used, the mixed valence compound is only obtained if it is trapped quickly, before isomerization of the meridional platinum(IV) complex to a facial form.544 Although these species nominally have localized valences, the extent of delocalization increases as the bridging halide... 

All compounds were measured at room temperature in air between Au electrodes inside a Faraday cage. IVT = maximum of the optical InterValence Transfer or intramolecular charge transfer band. RR Rectification Ratio. Survives cycling means RR does not decrease with cycling of IV measurements. AR or anti-AR indicates whether electron flow is from A to D (AR) or from D to A (anti-AR)... 

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. 

This has the very important consequence, as we show in more detail later, that no totally symmetric mode of a mixed-valence compound can contribute to the intervalence bandwidth (in the approximation of equal force constants in both oxidation states). For the moment we therefore drop the terms in Q+ and define the dimensionless variables... 

Binary Compounds. The thermodynamics of the formation of HfCl2, of HfCl4, fused sodium and potassium chlorides have been described. The reduction of ZrXj (X = Cl, Br, or I) with metallic Zr or A1 in molten AICI3 has been studied at temperatures from 250 to 360 °C, depending on the halide. The electronic spectra of the initial reaction products were consistent with either a solvated Zr complex or an intervalence Zr "-Zr" species. Further reduction resulted in the precipitation of reduction products which were identified by analysis and i.r., electronic, and X-ray powder diffraction spectra. The stability of the trihalides with respect to disproportionation was observed to increase from chloride to iodide thus ZrC and ZrCl2,0.4AlCl3 were precipitated, whereas only Zrlj was formed. ... 

Figure 10 Optical intervalence transfer in a symmetric mixed-valence compound. Correlation between spectroscopic and kinetic parameters.

These complexes show intense broad intervalence bands (Ptlv<—Ptn) in the electronic spectra. Irradiation within the intervalence band leads to strong intensification of the Raman band attributed to the totally symmetric stretching mode vsym(X—Ptlv—X) and its overtones. Data have been collected and analyzed for a wide range of these mixed valence compounds.1961-1971 These complexes are discussed more fully by Underhill in Chapter 60. 

An interesting new bridged complex, [Ru(TDBOHPP) L a L] (type H) in which the bridge L a L is 4,4 -azopyridine, has been studied in the search for molecular switches [217]. Protonation of the polymer induces partial oxidation of the Ru(II) to Ru(III) at the expense of the azo groups which are reduced to hydrazo species. Along with the formation of the Ru(II)-Ru(III) mixed valence compound a NIR intervalence band is switched on . The chemistry of these complexes is further complicated by the phenolic hydroxy groups in the porphyrin ligand which can also be deprotonated and oxidized. 

Models for the static spectral sensitization of coordination compounds are discussed. Among them, ion pairs with oppositely charged metal complexes, where intervalence charge-transfer interactions occur, appear to be promising exarrples. The spectra and the electron transfer kinetics,of mixed-valence ion pairs of ihe type M /fMCNjgJ4-, where M + = Fe3 Cu2 UO V02 are discussed in detail. 

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... 

In general, intensely-colored Pt compounds owe this feature to intervalence CT transitions, hence to the presence of Pt in different oxidation states. Provided the solid state structure is favorable, a tiny deviation (< 1 %6) from an integral oxidation number can be sufficient to cause the phenomenon of color. For example, the famous Magnus Green salt, usually formulated as... 

The charge transfers in polynuclear complexes with metal centres ready to undergo redox reactions are called intervalence charge transfer (IT). These compounds are often characterized by low-energy M-M transitions and thereby have intense colour (eg Prussian blue and its analogues). 

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