Tetrad effect lanthanides

What became known as the tetrad effect was first observed in the late 1960s during lanthanide separation experiments [25]. Fig. 1.3 shows a plot of log K, where is the distribution ratio between the aqueous and organic phases in a liquid-liquid extraction system. There are four humps separated by three minima, first at the f /f pair, secondly at the f point, and thirdly at the pair. 

Very often, the tetrad effect is not clearly discernible in the energies of processes in which 4f electrons are conserved. It may, for example, be obscured by irregularities caused by structural variations in either reactants or products. This is especially likely given the willingness of lanthanide ions to adopt a variety of coordination geometries. There is, however, no doubt that tetrad-like patterns are often observed. But does Table 1.2 provide a convincing explanation of what is seen ... 

These have been calculated from Caro s spectroscopic analyses [35]. The ligands come from opposite ends of the nephelauxetic series, so for a lanthanide reaction, A rep(irteg) should be relatively large. Even so, although it proves to be the largest contributor to the overall change, AEqs and AEso are significant Quantitative analyses of claimed examples of the tetrad effect must take such terms into account... 

Fig. 6a-c. Different ways of showing the nephelauxetic effect using the lanthanides for the formation of the chelate MA3 (HA=TTA) and the citric complex MHCit Cit2-. q = number of f electrons, L = orbital quantum number, a) double-double effect (23) b) tetrad effect (24) c) W inclined plot (25). 

The contraction of the actinides, as measuredby changes, with atomic number, of the unit cell volume of their compounds in oxidation states III, IV, and VI, exhibits the same tetrad effect as that observed in the corresponding lanthanides. 

Deviations from regular smooth variation of properties of lanthanides occur at quarter-, half- and three-quarter filled 4/ configurations which have been attributed to tetrad effect. This effect has been attributed to small changes in Racah parameters when the ligands around the metal change during the reactions. The half-filled shell effect and the quarter-and three-quarter shell effects are caused by changes in El and 3 in the theoretical ionization potential expressions for /" ions [4],... 

Tetrad Effect of Lanthanide Elements - Changing Gradation Rules in Lanthanide Coordination Chemistry... 

Because of gradation filling of electrons into the 4f shell, the properties of many lanthanide compounds show changing gradation with an increase in the atomic number. The lanthanide tetrad effect is an important phenomenon and has also been well studied. Because the separation of lanthanide elements was required before a study of the properties of individual lanthanide elements was possible, the discovery of the lanthanide tetrad effect was related to the separation of lanthanide elements. 

It was found that when extracting lanthanide elements with tributyl phosphate at low pH, IgD-Z showed an odd-even effect, which is observed when plotting the logarithm of distribution coefficient D versus the atomic number Z. Straight lines are plotted when Z is odd or even but the odd line is above the even one. Since this report, a lot of data have been reported and presented differently. Figure 1.14 shows typical curves for the change in lanthanide gradation. The lanthanide tetrad effect will also be very clear if the y-axis is not log D but Ig ex... 

To discuss the tetrad effect quantitatively, Nugent analyzed lanthanide and actinide elements using the approximate electronic repulsive energy equation proposed by Jprgensen [15]. He suggested that the electronic repulsive energy between the electrons of the f configuration is related to the electron number q. In fact, the macro tetrad effect is a representation of the relationship between and q. 

Reprinted from Journal of Inorganic and Nuclear Chemistry, 32, L.J. Nugent, Theory of the tetrad effect in the lanthanide(III) and actinide(III) series, 3485-3491, 1970, with permission from Elsevier). 

Figure 1.16 The ground state electronic repnlsive stahilization energy 1 as a fnnction of the 4f electron nnmher q (the contribution from the F term in Eqnation 1.3, solid hne the contribntion from the term in Eqnation 1.3, dashed line) [15]. (Reprinted from Journal of Inorganic and Nuclear Chemistry, 32, L.J. Nngent, Theory of the tetrad effect in the lanthanide(lll) and actinide(lll) series, 3485-3491, 1970, with permission from Elsevier.)...

It should be pointed out that not all the ions discussed here are affected by the outer fields. In fact, lanthanide ions may be affected by solvents or coordination fields in chemical reactions. For example, E and E will change because of the coordination effect of water or organic molecules in an extraction. In addition, the amount of change would be different in different media. The tetrad effect would thus be different in different systems. The tetrad effect not only relates to the electronic configurations of lanthanide elements but is also affected by the surrounding conditions. Currently it is still not possible to predict the tetrad effect or to calculate it quantitatively. Tetrad effect theory still needs to be improved and further data need to be accumulated. 

Bau M. (1999) Scavenging of dissolved yttrium and rare earths by precipitating iron hydroxide experimental evidence for Ce oxidation, Y-Ho fractionation and lanthanide tetrad effect. Geochim. Cosmochim. Acta 63, 67-77. 

Davranche. M. et al.. Adsorption of REE(III)-humate complexes onto MnOj Experimental evidence for cerium anomaly and lanthanide tetrad effect suppression, Geochim. Cosmochim. Acta. 69, 4825, 2005. 

It should be remarked here, that the tetrad effect is really not a new one" (our italics), and can be traced back to Endres (48) who divided the whole of the lanthanide series into four subdivisions, very much similar to those in the modern tetrad series. The paper of Fidelis and Siekierski (46) prompted Rowlands (49) to review the regularities . He plotted some 21 plots for 15 ligands and observed that the regularity proposed was based on insufficient data and this leaves doubt as to its general validity . Although the phosphoric acid systems exhibit the regularity, the same correlation could not be obtained for several other ligands. 

Jtfrgensen (55) and Nugent (56) quite independent of each other tried to provide a theoretical explanation for the tetrad effect in the lanthanide and actinide series. Both authors have emphasized the importance of the change in Racah parameter E3. 

Thus it seems that more than a passing reference is appropriate as to the relation of L-quantum number and the tetrad effect, and indeed Sin ha (58) has recently shown that the L-values exhibit the same periodicity as the tetrad effect in the lanthanide (actinide) series (Fig. 6), as well as that the properties of the/-transition metal ions vary linearly within each tetrad (58). It is a great pity that Klemm did not divide the lanthanide series based on the repeatation of the total angular momentum (L-values) values (see the above table), rather he showed and supported the classification of Endres (48). 

The tetrad effect, however, should not be ignored. There is a difference between explaining chemistry by resorting to tetrads and exploiting the observed tetrad effect to efificiently separate adjacent lanthanides. Purely electrostatic bonding models form an adequate foundation for describing the solution chemistry of rare-earth cations, but intralanthanide separations are performed as a function of atomic number, not ionic radius. The variations in the intra-lanthanide separation factors that create the breaks between tetrads in fig. 14a are real and can be exploited in separations even if the immediate cause is electrostatic. 

The tetrad effect has important implications for the mutual separation of lanthanides(III), and presumably actinides(III), since in even the plots with no maximum the beginning of a tetrad differs markedly from the end of the same tetrad with respect to r, the ratio of Kz+i/Kz- In curve (A) of Fig. 6, for example, the r for the Ce-La pair is 7.8 while that for the Nd-Pr... 

A number of studies have reported a tetrad effect for the geochemical behaviour of the lanthanide series, including stability constants and distribution coefficients (Kawabe, 1992 Kawabe and Masuda, 2001 Ekberg, Englund and Persson, 2012). Kawabe and Masuda (2001) utilised refined spin-pairing energy theory (RSPET) to describe the tetrad effect on distribution coefficients. The RSPET equation is derived from the Slater-Condon-Racah theory that has been applied to free Ln " ions in a vacuum (Kawabe, 1992). As applied to the stability/solubility constants of the lanthanide metals, the RSPET equation has the form... 

The stability constants determined for the first monomeric hydrolysis constant of the lanthanides demonstrate the tetrad effect, as is illustrated in Figure 8.9. The use of Eq. (8.1) with the first monomeric constants of the lanthanide metals shows the apphcabihty of the equation. The majority of the predicted... 

Kawabe, I. (1992) Lanthanide tetrad effect in the Ln ionic radii and refined spinpairing energy theory. Geochem. )., 26, 309-335. 

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