Intervalence bands

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... 

Halide bridged nickel(II)/platinum(IV) and palladium(II)/platinum(IV) complexes with onedimensional linear-chain structures have been studied extensively.556-559 These are analogous to the linear-chain platinum(II)/platinum(IV) systems described in Section 6.5.13 and have similar broad and intense intervalence bands.556-559 However, they have weaker metal-metal interactions resulting in more localized valences than the all platinum analogues.556-559... 

Table III also includes values of Hab and k estimated from the properties of the intervalence band observed for...

A vibronic coupling model for mixed-valence systems has been developed over the last few years (1-5). The model, which is exactly soluble, has been used to calculate intervalence band contours (1, 3, 4, 5), electron transfer rates (4, 5, 6) and Raman spectra (5, 7, 8), and the relation of the model to earlier theoretical work has been discussed in detail (3-5). As formulated to date, the model is "one dimensional (or one-mode). That is, effectively only a single vibrational coordinate is used in discussing the complete ground vibronic manifold of the system. This is a severe limitation which, among other things, prevents an explicit treatment of solvent effects which are... 

To illustrate some of the characteristics of the two-mode model, we calculate the intervalence band of the Creutz and... 

We now show that the two-mode model can account for the observed intervalence band asymmetry of the C T ion, the weak tunneling transitions and a Ar value of 0.04 8. We fix the... 

Fig. 4) is physically more reasonable. As a practical matter, as k is increased, the calculation becomes increasingly unreliable because of the severely truncated basis set. The important point is that inclusion of a low frequency effective solvent mode can nicely rationalize the observed characteristics of the C T ion. Similar improvements may be anticipated in analyzing intervalence bands in other systems. 

It has recently been suggested by Hush (13) that the asymmetry on the high energy side of the C T ion intervalence band can be easily rationalized by including contributions from totally symmetric modes of the ion. Day (20) has interpreted the... 

Figure 2. Experimental intervalence band (10) and calculated contour for the Creutz and Taube ion. The parameters used in the calculation are c, —6.0 Ac, 1.1 Ag, 5.51 vc, 500 cm"1 and vB, 100 cm"1 at 300 K. The sticks show the relative calculated intensities, and the contour is obtained by replacing each stick by a Gaussian of FWHM = 1.4 vc. (Some of the vertical bars—in Figure 2 only— have been omitted for clarity.) The experimental peak height is normalized to the calculated value, which is arbitrary. The calculated relative intensities among Figures 2-4 are meaningful.

The position of the intervalence band is measured by the mean band energy E ... 

DR. SCHATZ I think those systems are not simpler. Since the intervalence band is at an order of magnitude lower energy than the intra-unit electronic transitions, one can use first-order perturbation theory in treating the former, but not the latter. I am assuming that other electronic states do not mix in significantly i.e., it is just 2,3 and 3,2 as the electronic basis in the Creutz-Taube complex, for example. If other electronic states mix in, the model is dead. 

Figure 2. Intervalence band maximum (hvmax), thermal activation energy E, and overall energy E0 for weak-interaction electron transfer.

The Electronic Interaction Term. A question raised earlier was is the magnitude of J sufficiently small to justify the weak-interaction assumption This is a question which can be answered, at least for mixed-valence complexes, by interpretation of the oscillator strength of the intervalence band. In Table III, some data (24, 25) for the intervalence band of the... 

Does T differ significantly from unity in typical electron transfer reactions It is difficult to get direct evidence for nuclear tunnelling from rate measurements except at very low temperatures in certain systems. Nuclear tunnelling is a consequence of the quantum nature of oscillators involved in the process. For the corresponding optical transfer, it is easy to see this property when one measures the temperature dependence of the intervalence band profile in a dynamically-trapped mixed-valence system. The second moment of the band,... 

DR. PAUL SCHATZ (University of Virginia) If one tries to get additional width, for example, in the Creutz-Taube ion intervalence band by putting in a symmetrical mode, and one works through the arithmetic, one finds that there is no intensity unless the force constants are different in the two oxidation states. And, furthermore, unless they are very different, the amount of intensity obtained will be very, very small. So I m personally very skeptical of your calculation using the symmetrical modes to obtain additional width. I don t think it will work. 

In the spectra of [V203(pmida)2] and [V203(nta)2]3-, a new band around 1000 nm was assigned to the intervalence transition.814 The spectrum of [V9Ol6(btda)4]7 is the same in solution and in the solid.813 The band at 900 and a shoulder at 1020 nm corresponded to two components of the intervalence band. 

The intervalence transitions of doped [M(oep)(L-L)] m+ polymers (M = Fe, Ru, or Os and L-L = pz, dabco, or 4,4 -bpy) have been studied. The metal-metal coupling and the intensity of the intervalence transition increase in the order dabco < 4,4 -bpy < pz, and the energy of the intervalence band decreases in the same order. Within a series with the same bridging ligand, the strength of metal-metal coupling and the intensity of the intervalence transition increase in the order Fe < Ru < Os, and the energies of intervalence bands decrease in the same order. With [Os(oep)pz] m+, an intense and relatively sharp electronic transition is observed at 2000 cm 1 in the IR (563-565), which is reminiscent of a similar transition in [(NH3)5Os(pz)Os(NH3)5]5+ (Section IV,B,l,b). 

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. 

Under milder oxidation conditions a different product is formed (25, 26), best formulated as [Ni"(en)2Nilv(en)2Cl2]Cl4. The magnetic moment is reduced to 0.76 BM and there is an intense intervalence band... 

---