Reduction of the free-ion parameters

The reduction of the free-ion parameters has been ascribed to different mechanisms, where in general two types of models can be distinguished. On the one hand, one has the most often used wavefunction renormalisation or covalency models, which consider an expansion of the open-shell orbitals in the crystal (Jprgcnscn and Reisfeld, 1977). This expansion follows either from a covalent admixture with ligand orbitals (symmetry-restricted covalency mechanism) or from a modification of the effective nuclear charge Z, due to the penetration of the ligand electron clouds into the metal ion (central-field covalency mechanism). 

Nearly identical results were obtained also for Y202S Eu3+ (Liu et al., 1998a) and Gd2C>2S Eu3+ (Chi et al., 1998a, 1998b). In both cases the reduction of the free-ion parameters was about -0.40% for F2 and -0.20% for f up to pressures of 13 GPa and 14.8 GPa, respectively. Considering the two different covalency models, the authors concluded that the SRC model describes the nephelauxetic effect more reasonably than the CFC model. 

Fig. 5 Regions of stability of either one or the other spin state as a function of the ligand-field strength. The region of spin crossover compounds is indicated by the shaded area. For the calculation, the values of the Racah parameters were taken to be 75% of the free ion values. This corresponds to a typical reduction for iron(II) coordination compounds of the type under consideration... 

The rare-earth trichlorides belong to the best studied systems at ambient as well as at high pressure. Voloshin et al. (1975) studied the effect of pressure on the free-ion parameters of NdCl3-6H20 up to 2.3 GPa. They found reductions of -0.26%, -0.29%, -0.12%, and -0.25% for F2, F4, F6, and f, respectively. Such variations can be explained neither by the CFC nor by the SRC model alone. 

Thus, IQDq in CFT becomes Aoct in LFT. Moreover, the interelectron repulsion parameters are permitted to vary from their free-ion values (the Nephelauxetic effect) the orbital angular momentum operator required to compute magnetic properties is attenuated via a multiplicative constant—Steven s orbital reduction parameter, k—and some workers even advocate a separate reduction in the free-ion one-electron spin-orbit coupling parameters, Thus, LFT is simply a parametric version of CFT. 

It is convenient at this point to complete our discussion of the reduction in the free-ion value of the Racah parameter B which occurs on complex formation. This, it will be recalled, appears to be a consequence of the metal electrons being delocalized over a larger volume of space in the complex than in the free ion. It has proved possible to arrange ligands in a series such that, for a given metal ion, the B value required to fit the spectra of the [MLg]" ions decrease down the series... 

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... 

Using this model, a least squares refinement of the data Was performed, taking the free ion value for the spin-orbit constant ] , = 360 K) and k = 0.8 as orbital reduction parameter. We obtain Jj = —13,5 K and D = 3000 K + 200 K... 

Zhao et al. [ 144,166] proposed an approach based on symmetry restricted covalency [167]. Symmetry restricted covalency attributes covalency effects to the delocalization of d electron density onto ligand orbitals through a and rr molecular orbital formation between metal d orbitals and ligand s, p, and/or d orbitals [168]. The formation of molecular orbitals occurs between metal d orbitals and ligand orbitals of the same symmetry and varies with the coordination environment. The delocalization of d electron density that occurs upon molecular orbital formation leads to a reduction in the Racah B and C parameter values from the free ion values Bg and Cg. The reduction can be expressed by... 

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