Intervalence transfer bands

D. Intervalence-Transfer Bands for Mixed-Valence Oligo(ferrocenylene)s... 

Many of these complexes are deeply colored and their electronic spectra contain very strong intervalence transfer bands in the visible, due to electron transfer from B to A (Robin and Day, 1967). They also show magnetic coupling and they have semiconducting properties. They are stable in air and in acid solutions, but they dissociate in alkali. They can be reversibly oxidized, whereby B gives up an electron and goes to a higher oxidation state, and the intense color disappears. 

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... 

Besides the usual MLCT bands in the visible, these spectra show clear Ru(II) Ru(III) intervalence bands in the near-IR region. Analysis of the intervalence transfer band of the binuclear complex according to Hush theory (Section 1.3.1) yields H g = 2000 cm X 5700 cm AE = 2000 cm", and + AE) = 0.07, indicating again a... 

Except for the presence of a reduced bpy ligand, these excited states have the same sequence of oxidation states as the mixed-valence forms, and thus similar intervalence transfer bands are expected in the two cases. The observation of such bands in laser flash photolysis [71,76] confirms this expectation and reinforces the description of such systems, not only in their ground states but also in the excited states, in terms of localized electronic configurations. 

Intervalence transfer bands have been noted in a series of saturated spiro sulfur-bound ruthenium (III) (II) mixed-valence dimers [(NH3)sRuS C2n+2H4 S Ru(NH3)5] , wherc n = 2,3, or 4 indicates the number of spiro rings. Calculated electron transfer rates decrease with intermetal distances giving values of 8.0 x lo s at 11.3 A for n = 2, 4.9 x 10 s at 14.4 A for n = 3, and 3.5 x lO s at 17.6 A for = 4 indicating very effective tunneling even at 17.6 A with only a cr-bonded framework. 

In perchloric acid solution, oxidation of [Mo(CN)g] by VO2 is biphasic, consisting of parallel cyanide-bridged inner-sphere and outer-sphere pathways. Both pathways are catalyzed by [H ] which is presumed to coordinate to the molybdenum complex and ionic-strength dependencies indicate that [HNa2Mo(CN)g] and [HNaMo(CN)g] are the reactive species. Photoexcitation of the intervalence transfer band in the [Mo(CN)g] , CUaq ion pair leads to electron transfer and allows determination of the dark reaction rate. The oxidation of Mn04 by [Mo(CN)g] has also been examined, in alkaline media. The reaction is outer sphere as are the corresponding reactions of [W(CN)g] and... 

The mixed-valence ion has an intervalence charge transfer band at 1562nm not present in the spectra of the +4 and +6 ions. Similar ions have been isolated with other bridging ligands, the choice of which has a big effect on the position and intensity of the charge-transfer band (e.g. L = bipy, 830 nm). 

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. 

One experimental source of these essential data (for mixed-valence systems) is the intervalence transfer absorption band. The moments of the envelope (particularly the first and second moments) can be interpreted to yield values for x and (from a study of temperature dependence) for an effective coupling frequency. The study of effects of solvent variation can also lead to separation of x into intramolecular and environmental components. ... 

The effects of temperature on the shape of intervalence charge-transfer bands for the radical cations of bis(2-f-butyl-2,3-diazabicyclo[2.2.2]oct-3-yl)hydrazines that are bridged by 2,5-xylene-1,4-diyl, durene-l,4-diyl, naphthalene-1,4-diyl, biphenyl-4,4-diyl and 9,9-dimethylfluorene-2,7-diyl aromatic rings were studied by ESR. ... 

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