Intervalence transfer spectra

Some representative examples of optical charge transfer spectra of cyano-bridged complexes will be given in this section. Other cases will be shortly mentioned in other parts of this article, particularly in Section 4.3. 

In this species, several electronic states are relevant, with the following one-electron configurations ( represents the radical anion of the ligand)  

Of these excited states, types 1 and 2 would be present in the isolated components as well, but 3-5 are new states of intercomponent charge transfer type, characteristic of the polynuclear species as such. Transitions corresponding to the various types of excited states can be easily identified in the absorption spectmm of diis complex. The resolution of the absorption spectrum of the complex into various types of transitions is shown in Fig. 8. By selective oxidation or reduction of the various sites in the molecule, very clear spectral changes take place, making the attribution of the various types of transitions straightforward [66,67]. 

The band analysis for the intervalence transfer taking place across the cyanide bridge (transition of type 4) according to the Hush model (Section 1.3.1) provides the following parameters = 1800 cm X 8700 

Of particular interest from the spectroscopic point of view is the direct observation of remote MLCT (type 3) and of remote IT (type 5), indicating that sizable electronic coupling between remote redox sites is present in these systems. An analysis of the remote IT transition according to Hush model (Section 1.3.1) yields = 300 cm between the terminal metal centers 

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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... 

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 ... 

The copper(I) alkynyls displayed rich photochemistry and particularly strong photoreducing properties. The transient absorption difference spectrum of [Cu3(dppm)3(/X3-) -C=CPh)2]+ and the electron acceptor 4-(methoxycarbonyl)-A-methylpyridinium ion showed an intense characteristic pyridinyl radical absorption band at ca. 400 nm. An additional broad near-infrared absorption band was also observed and it was assigned as an intervalence-transfer transition of the mixed-valence transient species [Cu Cu Cu (dppm)3(/x3- -C=CPh)2] +. The interesting photophysical and photochemical properties of other copper(I) alkynyl complexes such as [Cu(BTA)(hfac)], 2 [Cui6(hfac)8(C=C Bu)8], and [Cn2o(hfac)8(CsCCH2Ph)i2] have also been studied. 

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... 

The absorption spectrum of a supramolecular system can differ substantially from the sum of the spectra of the molecular components. Aside from those small shifts that can be dealt with in terms of perturbation of the spectra of the single components upon bridging, some totally new bands can be present in the spectrum of the supermolecule. These bands correspond to optical electron transfer transitions, commonly denominated charge-tranter and intervalence transfer transitions, in the organic [26] and inorganic [23-25] literature, respectively (process 7 in Fig. 4, eq 1). The factors that determine the spectroscopic characteristics of such bands... 

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. 

Give the systematic name of Pmssian bine. In what region of the visible spectrum does the intervalence charge transfer absorb light ... 

Figure 2. Optical spectrum of biotite BIO, replotted from Robbins and Strens (1972), with the beam polarized parallel to and perpendicnlar to the cleavage direction. The peaks at 870 and 1100 nm correspond to bnt the assigiunent of the band at about 700 mn is controversial. Robbins and Strens (1970) assigned this band to Fe -Fe intervalence charge transfer, an assigiunent snpported by Smith et al. (1980) and Riischer and Schrader (1996). However, Kleim and Lehmann (1979) assigned it to single ion Fe band.

The main difference in the spectra concerns the presence of an intense absorption band at around 700 nm, which can be attributed to Intervalence Charge-Transfer between Mo5+ and Mq6+ in the Keggin unit [22,23], thus indicating the presence of a partially reduced POM for the catalyst which has been unloaded after reaction under isobutane-rich conditions (spectrum... 

An intervalence electron-transfer band, not present in the Fe analogue, was observed in the room temperature electronic spectrum at 13 800cm . Mossbauer spectra indicated distinct Fe" and Fe " sites at 17K while at 300 K a single absorption was observed. The thermal barrier to electron transfer in the trimer was estimated as about 470 cm. Triiron clusters of this type, in the presence of zinc powder, acetic acid, aqueous pyridine and oxygen, are reported " to effect the oxidation of saturated hydrocarbons. The exchange interactions in the series of complexes [Fe2 M 0(02 CMe)spyj] py (M = Mg, Mn, Co, Ni or Zn) which, M = Ni excepted, are isomorphous with the mixed valence complex referred to above have been measured. 

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