2:3 lanthanide complexes stability constants

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

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. 

The most efficient separations will be achieved when the lanthanide R is more strongly transported to the counterphase (RX3) and more weakly complexed by the aqueous complexant (L ) (or vice versa) If we simplify this relation by eliminating some fractions and substitute the complexation equilibrium constants, including that for the phase transfer equilibrium (Xgx), this expression becomes a relatively simple function of the extraction equilibrium constants for the metal ions (Kgx), the complex stability constants, and the free ligand concentration (recognizing that the free ligand concentration carries a pH dependence as well) ... 

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. 

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. 

In contrast to the situation observed in the trivalent lanthanide and actinide sulfates, the enthalpies and entropies of complexation for the 1 1 complexes are not constant across this series of tetravalent actinide sulfates. In order to compare these results, the thermodynamic parameters for the reaction between the tetravalent actinide ions and HSOIJ were corrected for the ionization of HSOi as was done above in the discussion of the trivalent complexes. The corrected results are tabulated in Table V. The enthalpies are found to vary from +9.8 to+41.7 kj/m and the entropies from +101 to +213 J/m°K. Both the enthalpy and entropy increase from ll1 "1" to Pu1 with the ThSOfj parameters being similar to those of NpS0 +. Complex stability is derived from a very favorable entropy contribution implying (not surprisingly) that these complexes are inner sphere in nature. 

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... 

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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