Nephelauxetic factor

Indeed, the graphs of isotropic chemical shifts versus the quotients of nephelauxetic factors and the d-d transition energies show a better correlation than the conventional FMR plot for Co(III) pentaamine and Co(III) pentacyano complexes. ... 

The value of 6 for metal ions in coordination compounds is always lower than the value of 6 that the metal has as a free ion, a fact known as the nephelauxetic effect, which derives from the German word for expanding cloud. Because metal-li d bonds have a covalent character, the effective size of the d-orbitals is larger in a transition metal complex than it is for the free ion. The metal electrons are somewhat delocalized over the metal-ligand bond. As a result, the value of 6 for the metal, which is a measure of the electron-electron repulsions, will always be smaller in a coordination compound than in the free ion. The nephelauxetic factor, is defined as the ratio of the Racah parameter in the coordination compound (6 ) versus that for the free ion (6), as shown in Equation (16.24)... 

Analysis of spectra is generally made to obtain (i) crystal field, (B ), (ii) nephelauxetic, (fi) and (iii) intensity parameters for a complex compound. These parameters are used in conjunction with relationships between spectroscopic, energy and geometry factors and deduce information on the structure of the complex. 

For the same ligand, the nephelauxetic effect and the crystal field depend on the following factors ... 

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) ... 

I ( Aig I Lz I Tig) 1 are collectively known as the covalency or the nephe-lauxetic factor. The classical Freeman-Murray-Richards (FMR) approach emphasizes the dependence of shielding on excitation energies, whereas the independent investigations by Juranic and Bramley demonstrated the importance of the nephelauxetic effect in Co shielding variation. On the one hand, Taura convincingly showed that the solvent dependence of the Co chemical shift for K3Co(CN)6 arises mainly from the solvent shift of the Tig transition. " The Co chemical shifts of a wealth of Werner-type complexes were successfully rationalized in terms of empirical... 

In contrast to the MF series there is abundant evidence that the hexafluoro complexes of Co and Ni, CoF and NiF, are both low-spin. This seems to be due to a combination of two factors in the first place a substantially greater value of T)q (ca. 2000—2200 cm. i) is found for the tetravalent cations than for the M(lll) series, and secondly the much greater degree of covalency which is found for the metal-fluorine linkages, as evidenced by the nephelauxetic ratios observed. These latter are substantially smaller than the corresponding values for the MF ions (s. Sect. 4) and thus lead to Dy/B values which lie on the strong field (low-spin) side of the crossover point. As for the trivalent series the trends in the observed / values are fully discussed elsewhere (Section 4). 

Such studies, which give direct information on the distribution of electrons in specific valence orbitals, have rarely been performed in the required detail to give quantitative information on bonding. The form factor of single crystal antiferromagnetic NiO< > showed an expansion which fitted closely with the free ion model including overlap and orbital effects. Indeed, the measurement of the covalently reduced moment and form factor expansion for this compound preceded the understanding of these effects. Early measurements on several polycrystalline Mn + compounds indicated a form factor contraction< > which was taken to be a reflection of the nephelauxetic effect—the expansion of the... 

TABLE 16.25 Jorgensen s h and k factors for the nephelauxetic series of ligands and metal ions. 

Phosphors are composed of the host lattice and the activator, which are discussed in detail in the following section. The optical properties of phosphors are mainly affected by several effects including the nephelauxetic effect, crystal field splitting, energy transfer, and thermal effect. These factors must be considered in practical and actual applications. We also list the suitable phosphors combined with UV- or blue-chip sources that have been developed and extensively used in lighting according to the excitation spectra. Other important factors, such as intensity (quantum efftciency), durability, thermal stability, and bandwidth, are discussed for the representative applications of phosphors. 

Figure 1.7 illustrates the overall factors of the host lattice affecting the emission properties of 5d 4/ transitions [3]. The nephelauxetic effect is the result of the covalency of the host lattice and ligand polarization. The polarization of nitrogen... 

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