Intervalence spectra

Figure 8. Intervalence spectrum of Creutz-Taube ion (------) and of the corresponding ion with 1-methoxypyrazine (-----) instead of pyrazine (DtO solution,...

However, the solution-phase value should still yield the overall value of x Thus, one can, in principle, find x from a one-center spectrum, as distinct from the two-center intervalence spectrum. Interesting work in this area, which will lead to such determinations, has been pioneered by Delahay (51). 

The case isovalence-electronic with BaBiOj is K2SbClg, a salt referred to as Setterberg s salt, which was discussed in Chapter 10. The average oxidation state for Sb in the salt is -1-4, but this is a missing valence state. Absorption, giving the strong color, is ascribed to Sb +Sb + Sb" +Sb" + by Day et al. The understanding of the intervalence spectrum is based on the local model. 

To summarize, it is thus possible from the analysis of a single intervalence spectrum... 

Mention should be made here of recent attempts by Piepho, Schatz and Krausz (46) to give a general interpretation of intervalence bandshapes in terms of a Hamiltonian equivalent to that of eq 6. They use vibronic eigenfunctions (following the method of solution of Merrifield (47)) rather than adiabatic Born-Oppenheimer (ABO) functions. Thus, the aim is to interpret an observed spectrum in terms of one vibrational coupling mode, which is antisymmetric. Their analysis of the spectrum of the Creutz-Taube ion yields a value of 0 of 1.215, i.e., a rather weakly localized ground state. Using their assumed unperturbed... 

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. 

In the extreme class III behaviour,360-362 two types of structures were envisaged clusters and infinite lattices (Table 17). The latter, class IIIB behaviour, has been known for a number of years in the nonstoichiometric sulfides of copper (see ref. 10, p. 1142), and particularly in the double layer structure of K[Cu4S3],382 which exhibits the electrical conductivity and the reflectivity typical of a metal. The former, class IIIA behaviour, was looked for in the polynuclear clusters of copper(I) Cu gX, species, especially where X = sulfur, but no mixed valence copper(I)/(II) clusters with class IIIA behaviour have been identified to date. Mixed valence copper(I)/(II) complexes of class II behaviour (Table 17) have properties intermediate between those of class I and class III. The local copper(I)/(II) stereochemistry is well defined and the same for all Cu atoms present, and the single odd electron is associated with both Cu atoms, i.e. delocalized between them, but will have a normal spin-only magnetic moment. The complexes will be semiconductors and the d-d spectra of the odd electron will involve a near normal copper(II)-type spectrum (see Section 53.4.4.5), but in addition a unique band may be observed associated with an intervalence CuVCu11 charge transfer band (IVTC) (Table 19). While these requirements are fairly clear,360,362 their realization for specific systems is not so clearly established. 

The difficulties of interpretation presented by some of these complexes are well illustrated by the sequence of ferrocene derivatives (VIII to X). The spectrum of VIII shows a near-infrared absorption as expected for a mixed valence complex, but the band has a shoulder, and at low temperatures is resolved into two bands. Complex IX has a well-defined band almost certainly ascribable to intervalence transfer, but complex X with a similar molecular structure has no such... 

The highly unstable, active titanocene 8, reported by van Tame-len et al., also forms a blue complex with dinitrogen, [(CjH Ti] (25) (Section III,C). Interestingly, the visible absorption spectrum of this titanocene 8 is identical with that of -(rj1 T75-CjsH4)(i -CsHs)3Ti2 (10) (bands at 486 and 640 nm) (3). Unfortunately however for comparative purposes, the reported spectrum of 8 does not cover the near infrared region at —1120 nm where 10 shows a characteristic intervalence absorption band. 

Similar oxidative coupling [Cu(OAc)2/pyridine/dbu, 40 °C] of Fe(C=CC=CH)(dppe)Cp gives Fe(dppe)Cp 2 r-(C=C)4 (159) as an air- and thermally-stable burnt-orange compound.307 Oxidation with [FcH]+ gave forest-green [ Fe(dppe)Cp 2( r-C8)]+ which has two v(CC) absorptions at lower frequencies (1784, 1879 cm-1), intermediate between the diyndiyl and cumulenic forms. The UV/vis spectra contain MLCT bands at 389, 481 nm (neutral) and 403, 423, 556, 624, 707 nm (cation), while the near IR spectrum contains a narrow intense intervalence CT band... 

Studies of the intervalence charge-transfer spectrum in solid-state inorganic materials and dinuclear mixed-valence complexes have given analogous information, less controversially. At low temperatures, the variation of spectral bandwidth is given by ... 

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