Nephelauxetic parameter

Table 18.7 Nephelauxetic Parameters for Metal Ions and Ligands. ...

From this equation, we see that the Racah parameter for the metal ion in a complex, B, is the value for the gaseous ion reduced by a correction (expressed as Bhk) for the cloud-expanding effect. As should be expected, the effect is a function of the nature of the particular metal ion and ligand. As a result, analysis of the spectrum for a complex must be made to determine both Dq and B (which is actually B for the complex). Table 18.7 shows values of the nephelauxetic parameters for ligands and metal ions. 

Of course these generalizations must be used with caution however, they are useful in comparing similar compounds. For a series of octahedral Cr(III) complexes it has been found that the oscillator strength, /, varies approximately linearly with the nephelauxetic parameter, j3, also a measure of covalency. ... 

A2g transitions, respectively. Besides being able to correlate jS with oscillator strength,/, of transitions the values may be employed 42) to evaluate the effective charges on the Cr(III) ions in various complexes. Fig. 4 provides plots of the effective metal charge vs. /3, the nephelauxetic parameter and B, the Racah interelectronic repulsion parameter for chromium(III). Jorgensen... 

TABLE I. Wave Numbers in kK (1000 cm-1) and Spectrochemical and Nephelauxetic Parameters. 

Nephelauxetic parameter (ft) and SINHA s covalency parameter of (6) for the HMPA-complexes of the lanthanides, and a comparison with other complexes containing P -> O donor ligands. TBP = tributylphosphate, OMPA = octamethylpyrophosphoramide, HMPA = hexamethylpyhosphoramide. To emphasize the weak nephelauxetic effect produced by the P-0 donor ligands, some of the fi and S values are expressed with three or four places after the decimal without rounding off. Ref. [290]. 

Pr(Tp)3] has been reported, structurally characterized, and its absorption and luminescence spectra discussed. The global field experienced by Pr3+ and the individual ligand field strength due to the Tp ligand, as also nephelauxetic parameters were determined.605... 

We shall now summarize recent works performed in our laboratory in order to establish the connection between absorption and emission maxima, quantum efficiencies and the nephelauxetic parameters with the electronegativity and composition of the host glasses. 

Jorgensen (66) proposed that the nephelauxetic effect results from an expansion of the partly filled shell due to the transfer of the ligands to the core of the central atom. The nephelauxetic parameter ft may be defined as,... 

The different origin of the h parameter and the electronegativity is emphasized in Fig. 10 in which plots of these values versus the nephelauxetic parameter are given. 

The basic strategy for the generation of one particular helicate therefore relies on three crucial factors (1) a judicious match between the intrinsic information borne by the metal (size, charge, polarizability, ligand-field stabilization) and the ligand strands (denticity of the binding units, steric constraints, type of donor atoms, nephelauxetic parameters) ... 

U.v.-visible spectral studies indicate that the ligand generally forms a chelate structure (35) with four-membered rings, and that the crystal field of Se is weaker than S. However, nephelauxetic parameters indicate that replacement of S by Se leads to a slight increase in the covalent character of the M-chalcogen bond. Another interesting paper describes the synthesis of the first selenocar-bamato complexes ... 

The data reported in this review enable an extensive compilation to be made of nephelauxetic parameters for MF and MFe complexes of the 3d series. Since we are here dealing directly with a series of anions, all octahedral or nearly so, for a constant ligand, F, it is possible to establish with some degree of confidence a nephelauxetic series for the M3+ and M + cations of the 3d block, and to examine the relationship of... 

In Section 1 (ii) we also explained the origin of the three nephelauxetic parameters, /S33, and fias. This whole topic has been exhaustively treated by Jorgensen 3, 115), but briefly, for octahedral systems,... 

The electron repulsion in a free ion in the gas phase is always larger than that in a complex and can be expressed by the nephelauxetic parameter p as the ratio of their Racah parameters ... 

After assignment of the bands the Hgand field parameters such as splitting, Racah and nephelauxetic parameters can be determined from the frequencies. As an example, with fully hydrated Ni(II)X zeolite, five bands are observed at 5100, 6850,8450,14,700 and 24800 cm from which the first two have to be attributed... 

The present author calculated the nephelauxetic parameter from the Eq. (9)... 

The electronegativity of different glasses is calculated from the position of the maxima of charge transfer bands of Eu in various glasses, and it has been shown that there exists a linear relation between the nephelauxetic parameters and optical electronegativity of glasses. 

---