Nephelauxetic effect, transition metal

At present there is consensus on the fact that the observed nephelauxetic effect in the spectra of lanthanide compounds is analogous to the phenomenon observed in the spectra of d-transition metal complexes. The nephelauxetic effect cannot be quantitatively interpreted by excluding the covalent interaction of lanthanide ions with surrounding ligands [34]. Jorgensen has proposed [38] two possible mechanisms of interaction for the observed nephelauxetic effect, namely (i) direct participation of lanthanide 4f orbitals in the formation of molecular orbitals also known as symmetry restricted covalency , (ii) transfer of some part of the ligand electron density to the unfilled 6s and 6p orbitals of the lanthanide also known as central field covalency . 

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. 

The numerical values of the interelectronic repulsion parameters, B and C, as determined from the spectrum of an actual transition metal compound (or at least the value of B ) are, in general, lower than the values in a free (gaseous) transition metal ion. This observation which was actually pointed out by several workers in the field (42, 43, 53, 60, 61), is called the nephelauxetic effect (19, 53). In a free 3d metal ion, the interelectronic repulsion integrals are approximately projwrtional to the average reciprocal radius of the partly filled 3d shell. The... 

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)... 

Lanthanide ions have been used as substitutes for Ca and Zn° in proteins to obtain information on the number of metallic sites (by simple titration) and on their composition. This may of course be extended to any molecule or materials. One very useful transition in this respect is the highly forbidden and faint Eu ( Do Fo) transition which is best detected in excitation mode by analyzing the emission of the hypersensitive transition Do — Fa since both the emitting and end states are ncm-degenerate, its number of components indicates the number of different metal-ion sites. Moreover, the energy of this transition depends on the nephelauxetic effect generated by coordinated atoms and ions at 298 K ... 

The order of ligand -field effect for the various anions is complex and is probably not obtainable from the type of data presented above as it is not known whether the anions are mono- or bi-dentate. The nephelauxetic effect as measured in these complexes is again complex. However, the results show unequivocally that the anions [CIO ], [CF COO] , and [BF ] derived from very strong acids Interact with transition metal cations in non-aqueous solvents of relatively low basicity. 

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