Lanthanide stability constants

Gadolinium anomalies have occasionally been conceptualized as a particular example of the tetrad effect (Masuda and Ikeuchi 1979, Masuda et al. 1987), wherein patterns in lanthanide stability constants are developed in response to enhanced stability of lanthanide 4f shells which are quarter, half and three quarter filled. In the recent analysis by McLennan (1994), the existence of a tetrad effect in natural systems was termed ambiguous. The stability constant analysis of Byrne and Li (1995) provided no prominent... 

The redox behaviour of Th, Pa and U is of the kind expected for d-transition elements which is why, prior to the 1940s, these elements were commonly placed respectively in groups 4, 5 and 6 of the periodic table. Behaviour obviously like that of the lanthanides is not evident until the second half of the series. However, even the early actinides resemble the lanthanides in showing close similarities with each other and gradual variations in properties, providing comparisons are restricted to those properties which do not entail a change in oxidation state. The smooth variation with atomic number found for stability constants, for instance, is like that of the lanthanides rather than the d-transition elements, as is the smooth variation in ionic radii noted in Fig. 31.4. This last factor is responsible for the close similarity in the structures of many actinide and lanthanide compounds especially noticeable in the 4-3 oxidation state for which... 

In view of the magnitude of crystal-field effects it is not surprising that the spectra of actinide ions are sensitive to the latter s environment and, in contrast to the lanthanides, may change drastically from one compound to another. Unfortunately, because of the complexity of the spectra and the low symmetry of many of the complexes, spectra are not easily used as a means of deducing stereochemistry except when used as fingerprints for comparison with spectra of previously characterized compounds. However, the dependence on ligand concentration of the positions and intensities, especially of the charge-transfer bands, can profitably be used to estimate stability constants. 

DeCarvalho and Choppin (10, 11) previously have reported the stability constants, complexation enthalpies, and entropies for a series of trivalent lanthanide and actinide sulfates. As their work was conducted a pH 3, the dominant sulfate species was S0 and the measured reaction was as in equation 12. 

Tablet. Stability constants for complex formation MA3, MHCitCit2 and MCit for trivalent lanthanides and actinides. Ionic strength 0.15...

We have considered typical examples of lanthanide and actinide solvent extraction by chelate formation, involving complexes with citric acid and with TTA, to prove that the labelling of a stable element by one of its radioactive isotopes can help to produce accurate data on the stability constants for complex formation. The method is applicable to elements with radioisotopes having a half-life allowing an ion concentration of 10 6m or less. Other methods of partition such as radiopolarography and radio-coulometry also result in accurate thermodynamical data when the same procedure of labelling is used (29). 

The stability constants for EDTA and DCTA undergo an increase in stability with increasing atomic number - a similar phenomenon has been observed for the lanthanide complexes (29). With DTPA, however, the stability constants undergo only a slight variation from Am(III) to Fm(III) such a phenomenon is also a characteristic of the equivalent lanthanide complexes. 

However, even this simplified formula does not justify the use of the ratio of stability constants of the extracted complexes as the only measure of selectivity of extractive separations. Such a widely used approach is obviously based on an implicit assumption that the partition constants of neutral complexes ML of similar metal ions are similar, so that their ratio should be close to unity. This is, however, an oversimplification because we have shown that the ifoM values significantly differ even in a series of coordi-natively saturated complexes of similar metals [92,93]. Still stronger differences in the values have been observed in the series of lanthanide acetylacetonates, due to different inner-sphere hydration of the complexes (shown earlier), but in this case, self-adduct formation acts in the opposite direction [100,101] and partly compensates the effect of the differences in. Tdm on S T(see also Fig. 4.15). Such compensation should also be observed in extraction systems containing coordinatively unsaturated complexes and a neutral lipophilic coextractant (synergist). 

The measurement of stability constants of complexes of yttrium, lanthanide, and actinide ions with oxalate, citrate, edta, and 1,2-diaminocyclohexanetetra-acetate ligands has revealed that there is a slight increase in the stability of complexes of the /-electron elements, relative to the others. A series of citric acid (H cit) complexes of the lanthanides have been investigated by ion-exchange methods and the species [Ln(H2cit)]", [Ln(H2cit)2] , [Ln-(Hcit)], and [Ln(Hcit))2] were detected. Simple and mixed complexes of dl- and jeso-tartaric acid have been obtained with La " and Nd ions, and the stability constants of lactate, pyruvate, and x-alaninate complexes of Eu and Am " in water have been determined. 

In a potentiometric study in propylene carbonate, using Pb11 or Tl1 as auxiliary ions, stability constants have been determined for a variety of crown ethers. Some results464 are shown in Table 8. They show that the wrap-around ligand dibenzo-30-crown-10 is relatively quite effective, while the 2 1 complexes, presumably of the sandwich type, are favoured for larger lanthanides and smaller crowns. 

In practice, the hypersensitive transitions are often used for the determination of stability constants in aqueous solution. Lanthanide absorption bands in solution do not normally change in position on complexation to such an extent that bands due to the complexed and uncomplexed ion can be clearly observed independently, as is often the case for d transition metal ions, but the marked change of intensity of the hypersensitive bands is sufficient to allow determination of K values, for example as demonstrated for various adducts of [Ho(dpm)3].6U... 

Svetlitski, R., Lomaka, A., Karelson, M. 2006. QSPR modeling of lanthanide-organic complex stability constants. Sep. Sci. Technol. 41 (1) 197-216. 

Stability Constants (log P aN, ) of Lanthanide Complexes with OOCMPO and CPw2, CPw3, CPw17, and CPn3 in MeOH (/ = 0.05 M) Determined by UV Spectrophotometry... 

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