Nephelauxetic shift

Further information about such complex formation has been obtained by spectroscopic methods, observing the nephelauxetic shifts in... 

A number of other U(III) complexes have been obtained by essentially the same method. U(teep)4(BPh4)3 is lilac-mauve and U(ompa)4(BPh4)3 is pink. The PFe salts are darker and redder in color. A red U(cmp)4(PF6)3 and a darker purple U(mbpo)4(PF6)3 have also been made (78). There is no structural or other information available on these compounds, beyond the observation of shifts in the vp=o oi" uq=o frequencies. The ligands are probably bidentate and the uranium 8-coordinate. They all show substantial nephelauxetic shifts (77). 

Neptunium(iii) was reported to be more stable towards oxidation than uranium(iii) when in aqueous solutions of complexing agents in the absence of oxygen. Nephelauxetic shifts for neptunium have been found to be less than for uranium in ter- and quadri-valent states. 

Chemical polarizability and Pearson softness 133 dp = nephelauxetic shift relative to R(lll) aqua ions... 

The energy levels of lanthanide free ions in crystals and solutions determined from the absorption spectra differ somewhat from those of gaseous Ln3+ ions and depend on the environment [33-35]. In the presence of ligand fields, the whole 4f" structure appears to shorten as compared to gaseous ions. The mean J level in Pr3+ shifts by 5 percent [33] and by one percent in Er3+ [36]. The red shift observed in the absorption spectra of lanthanides has been termed as the nephelauxetic effect. The nephelauxetic effect has been discussed in detail [34],... 

The phenomenon of the nephelauxetic effect cannot be interpreted in the framework of an electrostatic model which considers exclusively first-order crystal field effects. The observed band shifts could be due to second-order crystal field effects. However, the theoretical values of the second-order crystal field effects are of the order of 10 cm-1 or so [40,41] and cannot account for the experimentally observed band shifts, which in some cases are as high as 100 to 1000 cm 1. 

The above series in general is similar to the ligand nephelauxetic series observed in the case of d-transition metals. The greatest nephelauxetic effect has been observed in sulphides [46], cyclopentadienides [47], and oxides [48] of lanthanides. However, attempts to formulate a common and general nephelauxetic series for the lanthanide series have been futile using aquo ions as reference standards. In the case of lanthanide complexes with the same ligand in aqueous solutions, the absorption band positions of light and heavy lanthanides shift in different directions. This unusual behavior of complexes may be due to the differences in structure of aquo ions and the complexes. 

In efforts to explore the significance of the nephelauxetic effect, the possible relationship between the nephelauxetic absorption band shift and the structure of the complex has been investigated. The dependence of the nephelauxetic effect upon the coordination numbers in the complexes was recognized by Jorgensen and coworkers [49]. The nephelauxetic... 

The Eu(III) absorption band due to the transition 7Fo —> 5Do and the shifts in this band due to the nephelauxetic effect were studied for various fi-diketones. A correlation between the energy of 7Fo —> 5Do transition and the sum of Taft s constants [186] aRI + Rll has been established as shown in Fig. 8.42. 

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