Electron 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]. 

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)... 

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. ... 

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. 

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... 

Other examples are the 1,2-diaminopropane complex [Ptn(pn)2][PtIV(pn)2X2]4+ and [Pt(NH3)4][Pt(NH3)4Br2]4+. These compounds commonly show dichroic behavior, intense absorption polarized parallel to the chain, exceptionally strong resonance Raman spectra, and electrical conductance. All these can be associated with intervalence electron transfer. Finally, as well as Pt11 and PtIV, it is possible to have Pt2n anions in the same compound, as in the copper colored needles, [PtII(en)2][Ptrv(en)2X2] 4+[Pt I(H2P205)4X2]4-. 

The heptanuclear complex [Cu°(tren)]6[Mo (CN)g] (C104)g (204) is characterized by the presence of six noninteracting Cu(ll) ions that surround the central diamagnetic [Mo (CN)g]" fragment [Fig. 45(Z))]. Upon irradiation with blue light (406 15 nm) at lOK, the compound exhibits a gradual increase in magnetic susceptibility that saturates after 10 h of continuous irradiation [Fig. 70(a)]. This effect was explained as a photoinduced electron transfer from Mo(lV) to Cu(ll) ions, which is promoted by the irradiation into the intervalence MMCT band... 

The main spectroscopic consequence of the combined action of electron transfer and vibronic interaction is the occurrence of the so-called electron transfer optical absorption (intervalence band), which is shown by the arrows in Fig. 10. The shape and intensity of the intervalence band in the PKS model is defined by the ratio t /(v /cu). In the case of weak transfer the Franck-Condon transitions are almost forbidden, and at the same time, the Stokes shift can be significant. Therefore the MV dimers of Class I are expected to exhibit weak and wide intervalence bands. On the contrary, in the Class III compounds the Franck-Condon transition is allowed, and the Stokes shift is zero. For this reason, intervalence optical bands in delocalized MV dimers are strong and narrow. When the extra electron jumps over the spin cores in a multielecton MV dimer d" — > 1) [85-87] we are dealing with... 

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... 

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