The nephelauxetic ratio

The results for Cr34 and the 3d5 cations Fe3+ and Mn2+ show that it is possible to derive values of the Racah B parameter for transition metal compounds from absorption bands in their crystal field spectra, enabling comparisons to be made with field-free ion values. In all cases, there is a decrease of the Racah B parameter for the bonded cations relative to the gaseous ions, which is indicative of diminished repulsion between 3d electrons in chemical compounds of the transition metals. This reduction is attributable to electron delocalization or covalent bonding in the compounds. Such decreases of Racah B parameters are expressed as the nephelauxetic Greek , cloud expanding) ratio, p, given by 

Two mechanisms contribute to the decrease of Racah B parameters. First, lone pairs of electrons from the ligand may penetrate the 3d shell of the transition metal and screen its 3d electrons from the nucleus, thereby decreasing the effective nuclear charge experienced by the electrons and expanding the 3d shell. This mechanism is termed central field covalency. In the second mechanism, referred to as symmetry restricted covalency, delocalization of the trans- 

Ligands can be arranged in order according to the extent to which they reduce interelectronic repulsion, as gauged by the Racah B parameter or nephelauxetic P ratio. This order constitutes the nephelauxetic (or cloudexpanding) series, a limited version of which is 

Such an order is intuitively one of increasing covalent bonding characteristics of the ligands based on their polarizabilities. The nephelauxetic series departs significantly from the spectrochemical series described in 2.9.2, which is based on relative values of the crystal field splitting parameter, A [see eq. (2.19)]. 

By maintaining the ligand constant a nephelauxetic series for cations may be derived. For example, from absorption spectra of metal fluoride compounds, the order of decreasing nephelauxetic ratio is 

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In this survey the nephelauxetic ratios have been calculated for the neutral metallocenes using the free-ion B values appropriate for the corresponding M( ) oxidation state, and for the metallicenium species the free-ion B values are similarly those for the Af(III) oxidation state. In both cases the B values calculated by the J rgensen-Racah relationship, and listed by Allen and Warren (60) have been used. 

In our previous survey (7) of the hexafluoro complexes of the 3d series we discussed in some detail the information which may be derived from a consideration of the value of the nephelauxetic ratio, / (=i compiex/5gas). To obtain this parameter it is clearly necessary to have available values of the free-ion Racah parameter, B, and for the 4 d elements these may be obtained either from the compilation of Di Sifiio et al. (75) or by use of the Jorgensen-Racah relationship (76, 77). 

From the discussion earlier in this chapter, we know that a value of 16,000 cm-1 for A0 is typical of most complexes of a +3 first-row transition metal ion. For V3+, the free-ion B value is 860 cm-1, so the value 600cm-1 found for V3+ in the complex indicates a value of 0.70 for the nephelauxetic ratio, ft. All of these values are typical of complexes of first-row transition metal ions. Therefore, even though the identity of the bands may be uncertain, performing the analysis will lead to B and Dq values that will be reasonable only when the correct assignment of the bands has been made. 

Pressure variations of the Racah B parameter Although the nephelauxetic ratio is only a qualitative indicator of relative covalent bond character of transition metal ions, interesting trends have been established, nevertheless, for minerals containing the cations Cr3, Fe3+ and Mn2+, for which Racah B parameters are readily obtained using eqs (11.1) and (11.4) (Keester and White, 1968 Manning, 1970 Abu-Eid and Bums, 1976). 

The nephelauxetic ratio for the B-value obtained from eq. (5) is called Pss of reasons to be discussed below. Terms with the same S and symmetry type are called a, b, c,. . . ... 

The orbital splitting pattern for sandwich compounds is defined by two parameters Aj and A2 B and C are Racah parameters and (3 the nephelauxetic ratio. 

The magnitude of the nephelauxetic effect may be conveniently expressed as the nephelauxetic ratio, / . This is the ratio of a given interelectronic repulsion parameter for the metal ion in a complex to its value in the gaseous ion. It is sometimes possible to evaluate different / s for different sets of orbitals, e.g., the eg and the 9 orbitals of an octahedral complex, reflecting the different amounts of a and n covalence. Thus, from spin-forbidden... 

For Cs2KCuFe the charge-transfer transitions are found at 29.5 and 37.2 K., as compared with 32.0 and 37.0 in KsNiFa. The lower position of the first band in the Cu(III) complex is in accordance with the decreasing stability of the 3- - oxidation state, as reflected by the general fall off in jS, the nephelauxetic ratio (s. Sect. 4), towards the end of the first transition series, and is discussed further in Sections 4 and 5. 

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). 

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