Charge transfer bands, intensities

In view of the magnitude of crystal-field effects it is not surprising that the spectra of actinide ions are sensitive to the latter s environment and, in contrast to the lanthanides, may change drastically from one compound to another. Unfortunately, because of the complexity of the spectra and the low symmetry of many of the complexes, spectra are not easily used as a means of deducing stereochemistry except when used as fingerprints for comparison with spectra of previously characterized compounds. However, the dependence on ligand concentration of the positions and intensities, especially of the charge-transfer bands, can profitably be used to estimate stability constants. 

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). 

Figure 4-4. a) Spectrum of [CoC J and b) Spectrum showing intensity stealing from a charge-transfer band at higher energy. 

The second example in Fig. 4-4 shows how a (spin-allowed or spin-forbidden) band lying close to a charge transfer band may acquire unusually high intensity. We shall discuss charge-transfer bands more in Chapter 6. For the moment, we note that they involve transitions between metal d orbitals and ligands, are often fully allowed and hence intense. On occasion, the symmetry of a charge transfer state... 

S Fe charge transfer bands are more intense in oxidized than... 

In its reduced state, the paramagnetic Rieske cluster shows a temperature-dependent MOD spectrum composed of numerous positive and negative C-terms that originate from the 8 = 1 ground state. The MOD spectra lack the Fe" 8 charge transfer bands that are observed as intense negative bands between 300 and 350 nm and a posi-... 

Photo-oxidation or reduction is often found if the complex is irradiated in the charge-transfer bands (see above) photo-oxidation of the metal occurring if the transition is M - -L. Thus the photochemical generation from Ir(IV)Cl6 of a species active in forcing filaments of E. coli may well involve the photoreduction of Ir(IV) to Ir(III) since the intense bands in the visible spectrum of Ir(IV)Clcharge-transfer bands. A report has appeared of the photo-aquation of IrCl -(43). 

The diffuse reflectance spectrum of CsReFg was studied between 10 and 40 kK. by Brown et al. (32), but no clearly defined peaks could be recognised above a high background absorption. Since Laporte-allowed charge-transfer bands usually yield well marked intense absorptions it is reasonable therefore to conclude that these are absent below 40 kK. (See also Section 5). 

As has been noted above, [Fe(HB(pz)3)2] undergoes a color change from deep violet to white upon heating, a change that is clearly revealed in its electronic absorption spectrum, see Fig. 4. The 297 K spectrum is dominated by a very intense charge-transfer band centered in the ultraviolet region and a less intense band centered at 19,000 cm-1. These absorptions account for... 

The electronic spectra of these materials recorded in chloroform solution appear to be dominated by intense bands originating from internal ligand transitions, metal-ligand and ligand-metal charge-transfer bands, whose intensities change markedly with changes in the population of the two spin states [7]. 

Both [Fe(acpa)2]X and [Fe(bzpa)2]X (X=PF6, BPh4, N03 ) show reversible thermochromism in acetone solutions, which is typical for a change in electronic ground state of the Fe(III) ion. The electronic spectra show a temperature dependence of the intensities of the metal-charge transfer bands ascribed to the high spin (550 nm) and low spin state (700 nm) [155]. 

The diphosphinonickel appended porphyrazines 63a-63d show a five peak absorption spectrum that is similar to the parent, 60a. An additional peak observed at 300 nm is due to a Ni —> P charge-transfer band seen in other Ni(II) diphosphine complexes (123). The peak is more intense than those reported in the literature for small molecule analogues because there are four Ni-P units per porphyrazine. 

---