Stability constants of lanthanide

TABLE 4.10 Stability constants of lanthanide-nitrate complexes in various solvents. ... 

Fig. 4.20. Stability constants of lanthanide cryptates in propylene carbonate at 298 K and p. = 0.1 M (NettCKL). From data reported by J.-C.G. Biinzli, in Handbook on the Physics and Chemistry of Rare Earths, eds K.A. Gschneidner, Jr., L. Eyring, Vol. 9, Ch. 60, North Holland, Amsterdam, 1987.

Stability constants of lanthanides with simple inorganic ligands at 298 K. 

Because of lanthanide contraction, the radius of lanthanide ions decreases gradually as the atomic number increases, resulting in regular changes in the properties of lanthanide elements as the atomic number increases. For example, the stability constant of lanthanide complexes usually increases as the atomic number increases the alkalinity of lanthanide ions decreases as the atomic number increases the pH at which hydrates start to precipitate from an aqueous solution decreases gradually as the atomic number increases. 

Liquid-liquid extraction has proved a practical technique for the isolation of rare earths and yttrium as a group and for the isolation of individual members of this group, the separation being accomplished especially easily in instances in which a valence other than +3 might be employed. It has proved useful in the determination of stability constants of lanthanide and actinide chloride and nitrate complexes. And it has demonstrated the existence of mixed extracted entities such as Th(N03)(HY2)3. 

Fig. 39.4. Log of stability constants of lanthanide complexes as a function of ionic radius. Filled circles porcine trypsin, data from Epstein et al. (1974). Open circles nitrilotriacetate (NTA), data from Moeller and Ferrus (1962). Crystal ionic radii of R from Templeton and Dauben (1954). 

Since a substantial number of stability constants of lanthanide complexes in anhydrous solutions has been determined using the spectrophotometric L-method, an important remark has to be made. The basic equation upon which the method relies gives the apparent absorption coefiBcient e of a solution of mononuclear complexes as a function of the ligand concentration ... 

A practical application of the spectral intensity of hypersensitive transitions in lanthanide ions is the determination of stability constants. Bukietynska et al. (1977, 1981a,b) have evaluated the stability constants of lanthanide complexes from the changes of the oscillator strength of hypersensitive transitions as a function of the ligand concentration. The method was applied to acetate, propionate, glycolate, lactate and a-hydroxyisobutyrate complexes of Nd ", Ho and Er +. Bukiet5mska et al. claim that this spectrophotometric method is superior to the potentiometric method, since the best results for the latter method can be obtained only for the first complexation steps. The stability constants evaluated at several temperatures have been used also to calculate the thermodynamic quantities AG, AH and AS. 

Meshkova S. B., Z. M. Topilova, M. O. Lozinskii, D. V. Bolshoi. Periodicity of Variation of Stability-Constants of Lanthanide(III) Complexes with Fluorine Derivatives of Acetylacetone, Russ. J. Inorg. Chem., 40, 1296-1301 (1995). 

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. 

Stability constants of rare earths with a variety of macrocyclic ligands are presented in Tables 3.13 to 3.19 (see Appendix) and the associated references are given at the bottom of the tables. As is the case with noncyclic ligands, stability constants are determined by several different techniques. One should be careful when comparing the values, since different conditions and techniques might have been applied. Stability constants for the complexes of cations with macrocyclic compounds are often influenced by the relative sizes of the cations and the cavities of the macrocycles, and hence the macrocycles have specific selectivities to various cations. It is naturally expected that macrocyclic compounds also exhibit unique selectivities to lanthanides. 

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. 

Fig. 4.11. Stability constants of the 1 1 complexes between lanthanide trinitrates and 18-crown-6 in acetonitrile, as determined by NMR spectroscopy (redrawn from J.-C.G. Biinzli et al in The Rare Earths in Modem Science and Technology, eds G.J. McCarthy, J.J. Rhyne. H.B. Silber, Vol. 2, 1980, p. 99ff, Plenum Press, New York).

The important difference between the dissociation mechanism of LnMEDTA complexes and the acid catalyzed dissociation of LnEDTA is the formation of mixed LnMEDTA-Ac" complex. The stability constants of such mixed complexes are small (1 to 10) and this may explain why such mixed complexes do not show up in the kinetics of dissociation or exchange of LnEDTA complexes. Further, in the present case [LnMEDTA]0 association with an acetate anion should be more favourable than [LnEDTA]- with an acetate anion based on electrostatic theory. Reference to Table 7.14 shows that Km values increase with increasing atomic number (decreasing radii) for heavy lanthanides but are independent for La-Nd series. 

The second chapter deals with quantum chemical considerations, s, p, d and f orbitals, electronic configurations, Pauli s principle, spin-orbit coupling and levels, energy level diagrams, Hund s mles, Racah parameters, oxidation states, HSAB principle, coordination number, lanthanide contraction, interconfiguration fluctuations. This is followed by a chapter dealing with methods of determination of stability constants, stability constants of complexes, thermodynamic consideration, double-double effect, inclined w plot, applications of stability constant data. 

These values compare favorably with those reported by Kury at /x = 0.5 and 25 °C. The stability constants of the lanthanide fluoride complexes were obtained by comparing changes in the potential in cell C with those in that of cell B upon adding sodium fluoride solution. Owing to the insolubility of LnFg, only a few (often only six) aliquots of sodium fluoride solution could be added to cell C before precipitation interferred. Precipitation could be observed easily since upon its occurrence, the potential between cells C and A would drift rapidly to lower values. 

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