LANTHANIDE IONS stability constants

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. 

Thus the separation of the two lanthanide ions from each other depends upon their respective complex formation constants. The lanthanide whose stability constant is higher will desorb and elute from the column in preference to the lanthanide whose stability constant is lower. 

PCTA is a tetraazamacrocyclic ligand bearing a pyridine chromophore and three carboxylic functions. It offers seven potential donor atoms able to coordinate a lanthanide. The stability constant of 20.3 for (Eu )PCTA is acceptable in order to work in the presence of competing ions or ligands [101]. A well known complex 37 is based on a phosphonate equivalent of DOTA Tb(III)-3,6,9-tris(methylene phosphonic acid -butyl ester)-3,6,9,15-tetraazabicyclo[9.3.1]-pentadeca-l (15),ll,13-triene (Tb-PCTMB) [102,103] k azx. = 270nm, 8259 = 3,000cm X (H2O) = 4.98 ms [104] tot (H2O) = 0.51. As one could expect from a single pyridine chromophore the absorption is rather low and the absorption falls below the workable window (300-340 nm) Amax = 269 nm, 6259 = 4,600 cm ... 

Use of table 3a in calculations of lanthanide seawater complexation requires that the total inorganic carbon concentration of seawater (Cr) be partitioned into dissolved aqueous carbon dioxide (C02aq), bicarbonate (HCOj) and carbonate (COj ). Since only a small fraction ( 14%) of COf in seawater is in the form of free ions, with the remainder being ion-paired as NaCOj, CaCO and MgCO, use of table 3a constants requires an assessment of carbonate and bicarbonate ion-pairing equilibria. Table 3b shows a more convenient stability constant formulation wherein lanthanide carbonate stability constants are expressed in terms of total (free plus ion-paired) carbonate ion concentrations in seawater ([C03"]t)- These results can be compared with direct observations of europium carbonate complexation in synthetic seawater (Lee and Byrne 1994). The GdCOj and Gd(C03)2 formation constant results of Lee and Byrne (1994), expressed in terms of... 

While lanthanide phosphate and carbonate stability constants increase substantially between La and Lu, the complexation behavior of lanthanides with sulfate changes very little across the lanthanide series. This difference in complexation constant trends is consistent with inner sphere (COj ) versus outer sphere (SO ) complexation behavior (Byrne and Li 1995). Stability constants for lanthanide sulfate complexes at 25 C and zero ionic strength could be well represented as logso4l8i(M) = 3.60d=0.08. The recommended stability constants (25 C, 0.7 mol kg ionic strength) shown in table 5 are based upon the works of Spedding and Jaffe (1954) and Powell (1974). Following the activity coefficient estimates of Millero and Schreiber (1982) and Cantrell and Byrne (1987a), lanthanide sulfate stability constants, expressed in terms of free-ion concentrations, were calculated as log 504 1 = log SO4/3 - 1.67. 

The lanthanide chloride stability constants which are recommended for seawater speciation calculations are shown in table 8. The logci/3 values shown in table 8 were calculated from the data of Mironov et al. (1982) and are nearly identical to the logcii i values used by Lee and Byrne (1993c) to correct the fluoride stability constant data of Bilal and co-workers (Bilal et al. 1979, Bilal and Becker 1979, Bilal and Koss 1980, Bilal 1980, Bilal and Becker 1980, Becker and Bilal 1985) for chloride ion pairing. 

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. 

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). 

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. 

During the early sixties Thompson and Loraas (77) reported the formation of mixed complexes of reasonable stability (log K 3.0—5.3) between lanthanide—HEDTA and ligands such as EDDA (N,N -ethylenediaminediacetic acid), HIMDA (N-hydroxyethyliminodiacetic acid) and IMDA (iminodiacetic acid). This observation together with the remarkably large formation constants (72) for the bis-EDDA complexes [log A2 =4.73 (La) 8.48 (Lu)] suggested a coordination number larger than six for the tripositive lanthanide ions in aqueous solution, in view of the fact that mixed chelates of the t5q>e M (HEDTA) (IMDA) axe not formed when M =Co(II), Ni(II) or Cd(II). 

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... 

An examination of the structure of lanthanide(III) ion complexes in solution using nuclear magnetic resonance is made in considerable detail. The importance of hydration in both the inner and outer spheres is stressed. The structural data are then used in an attempt to understand the stability constants and the rates of exchange of bound ligands. Fluctional properties are also analysed. Finally, the importance of the structures, the stability constants and the kinetic properties are related to the effects of Ln(III) ions, and through them Ca(II) ions, in biological systems. 

These values reflect moderate interaction between Ln(III) and the chloride ion. The overall stability constants for the formation of chloro and bromo complexes of lanthanides (y6(MX)2+ and (MXj)) are given in Table 4.7. 

Interaction of the nitrate ion with lanthanide(III) in acetonitrile solution was studied by conductivity, vibrational spectroscopy and luminescence spectroscopy. Bidentate nitrate with approximate C2V local symmetry was detected. FT-IR spectral evidence for the formation of [La(N03)5]2, where La = Nd, Eu, Tb and Er with coordination number 9.9 has been obtained [128]. Two inequivalent nitrate ions bound to lanthanides were detected by vibrational spectroscopy. The inequivalent nature varied with different lanthanides. For example three equivalent nitrate groups for La and Yb, one nitrate different from the other two for Eu ion were detected. Vibrational spectral data point towards strong La-NC>3 interaction in acetonitrile [129]. Stability constants for lanthanide nitrate complexes are given in Table 4.10. 

Carboxylic and hydroxy carboxylic acids form complexes readily with lanthanides with high stability constants (Chapter 3) and they have been widely used in the ion exchange... 

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