Intervalence charge-transfer bands

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 

Class 2—clearly interaction, but chemical identity retained 

Class 3—clearly interaction, chemically distinct species cannot be identified 

The discussion in this Chapter has, at times, been somewhat complicated, the complications centring around such topics as the extraction of quantitative data from spectra or the details of how a forbidden band obtains intensity. The assignment of the band pattern has, tacitly, been assumed to be straightforward. This is by no means always so. For first row transition metal complexes there is seldom a problem but for the others the larger values of A mean that there is more overlap between d-d and charge-transfer bands. 

Further, the larger values of the spin-orbit coupling constants for these elements make intensity and band-width criteria for distinguishing between spin-allowed and spin-forbidden transitions less reliable. Even more difficult is the case of lower-than-octahedral symmetry. It is probably true that the electronic spectrum of no transition metal complex has been studied in as much detail as has that of the square-planar [PtC ] ion. Despite all of this work, it is only relatively recently that some general agreement on the assignment of the spectrum has been achieved and few would regard as completely improbable a revision of some aspect of the current interpretation. 

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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). 

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. ... 

Bocarsly, Pfennig and co workers reported interesting multi electron photoreac tions for the trimeric M" Ptlv M" complexes 25a-c.212 215 In this system, a single photon excitation into the intervalence charge transfer band results dissociation of the trimer into [Pt(NH3)4]2+ and two equivalents of a M111. The initial photoexcited complex is though to dissociate first to a Mm complex and Ptni-Mn intermediate. The latter dimer subsequently undergoes a thermal electron transfer reaction to yield the final products. 

Figure 9 A Fe -Fe intervalence charge-transfer band in the mineral vivianite, Fe3 (P04)3-8H20 upon oxidation. (Ref. 22. Reproduced by permission of Kluwer Academic Publishers)...

The monocations of 5 and 6 were prepared and their near infrared intervalence charge-transfer bands studied the dication of 6 was also obtained but, as suggested qualitatively by cyclic voltammetry, the trication was too unstable to isolated (12). 

These reactions can be attributed to the excitation of charge-transfer states. High-energy UV irradiation (A < 200 nm) leads to high-quantum-yield photoreduction of Ru(III) amine complexes. Another photoredox-induced substitution is the result of exciting the intervalence charge-transfer band of the ion pair. ... 

UV-Vis-NIR spectroelectrochemistry normally allows for a convenient identification of the intervalence charge-transfer band that is crucial for the analysis of electronic interaction, ... 

The interesting redox chemistry of such conjugated polymeric metallocene systems has been reviewed [121]. For example, oligo(l,ll -dihexylferrocenylenes) 61 show an intervalence charge transfer band (IVCT) on oxidation. For the singly oxidized chains, the IVCT band zmax decreases as the number of ferrocene units increases, as expected. Also, as expected the energy of the IVCT band increases as the oxidation level increases. 

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