Intervalence charge transfer transitions

In crystal field spectra, electrons excited between split 3d orbital energy levels remain on individual transition metal ions, although magnetic interactions 

Two different situations exist, depending on whether the same element (homonuclear) or two different elements (heteronuclear) are involved in the electron transfer. 

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Vivianite, Fe2+3(P04)2.8H20, is the classic example of a mineral showing an intervalence charge transfer transition (Wherry, 1918 Bums, 1981). Vivianite has a diagnostic indigo-blue colour and a well characterized Fe2+ —> Fe3+ IVCT absorption band in the polarized spectra illustrated in fig. 4.12 and is the datum with which electron interaction parameters for other minerals are compared. The chemical formula of vivianite is not indicative of a mixed-valence compound. However, the pale-green colour of newly cleaved vivianite crystals or fleshly... 

In the previous chapter it was shown how measurements of polarized absorption spectra in the visible to near-infrared region can provide information on such crystal chemical problems as oxidation states of transition metal ions, coordination site symmetries and distortions, cation ordering and the origins of colour and pleochroism of minerals. Much attention was focused in chapter 4 on energies of intervalence charge transfer transitions appearing in electronic absorption spectra of mixed-valence minerals. 

Table 9.3. Intervalence charge transfer transitions in minerals at high pressures...

Figure 2 Potential curves for a mixed-valence dinuclear complex (A+---A) as functions of the energy gap reaction coordinate X. Broken lines (a), no electronic interaction (b,b), (c,c), (d,d), successively stronger interactions, the last corresponding to Hab > A/2 (see text) so that the symmetrical structure is the most stable. The vertical arrow indicates an intervalence charge-transfer transition...

Figure 3 Plots of free energy of zero-order precursor and successor states versus reaction coordinate, for electron-transfer reactions / (A+- B) — r(A- B+). (a) AG° > 0 (b) AG° = 0 (c) 0 > AG° > —X (d) AG° < —X. The upward-pointing arrows in (a), (b), and (c) indicate intervalence charge-transfer transitions the downward-pointing arrow in (d) indicates a possible fluorescent transition from the precursor state / (A+- -B)...

Fig. 2.21. Polarized electronic (optical) absorption spectra of a vivianite crystal with zones (labeled 1, 2, 3) exhibiting three different degrees of oxidation. The arrows identify Fe + crystal-field bands at 8,300 and 11,400 cm , and the Fe + —> Fe intervalence charge-transfer transition at 15,800 cm (after Amthauer and Rossman, 1984 reproduced with the publisher s permission).

Figure 1.46). ° The monocation 97 can be considered as a push-pull chromophore and exhibits a / i.o6 (HRS) value twice that of its neutral precursor 97 and the fully oxidised compound 97. Complex 97 is a class II mixed-valence complex, retaining charge localisation, but with enough electronic coupling between the redox centres to permit intervalence charge transfer transitions. As the redox couples 97/97" and... 

Spectroelectrochemistry provides a convenient avenue to assess whether this conclusion is correct by allowing the isolation and spectroscopic smdy of the mixed valence state. Oxidation of the first metal center results in the formation of the mixed valence Ru(II)Ru(III) ion reflected in the grow-in of a new optical transition centered at 1700 nm (s = 2250 dm mol cm ) and identified as an intervalence charge transfer transition (IVCT). The optical characteristics of such transitions can be analyzed according to the Hush theory (21) and used to estimate the extent of electronic coupling between two metals across the intervening bridge. The full width at half maximum (FWHM) (Vj j) of the... 

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