Lanthanide cationic radii

If liquid chromatographic techniques are to remain the premier analytical methods for the analysis of samples containing multiple lanthanides, what improvements could be made The obvious suggestion is the development of reagents or processes that interact with greater sensitivity to the lanthanide cation radius than hiba. We have shown above some examples of the effect of ligand rigidity on the selectivity of a... 

However, solubility, depending as it does on the rather small difference between solvation energy and lattice energy (both large quantities which themselves increase as cation size decreases) and on entropy effects, cannot be simply related to cation radius. No consistent trends are apparent in aqueous, or for that matter nonaqueous, solutions but an empirical distinction can often be made between the lighter cerium lanthanides and the heavier yttrium lanthanides. Thus oxalates, double sulfates and double nitrates of the former are rather less soluble and basic nitrates more soluble than those of the latter. The differences are by no means sharp, but classical separation procedures depended on them. 

The coordination chemistry of the large, electropositive Ln ions is complicated, especially in solution, by ill-defined stereochemistries and uncertain coordination numbers. This is well illustrated by the aquo ions themselves.These are known for all the lanthanides, providing the solutions are moderately acidic to prevent hydrolysis, with hydration numbers probably about 8 or 9 but with reported values depending on the methods used to measure them. It is likely that the primary hydration number decreases as the cationic radius falls across the series. However, confusion arises because the polarization of the H2O molecules attached directly to the cation facilitates hydrogen bonding to other H2O molecules. As this tendency will be the greater, the smaller the cation, it is quite reasonable that the secondary hydration number increases across the series. 

Various crown ethers (p. 96) with differing cavity diameters provide a range of coordination numbers and stoichiometries, although crystallographic data are sparse. An interesting series, illustrating the dependence of coordination number on cationic radius and ligand cavity diameter, is provided by the complexes formed by the lanthanide nitrates and the 18-crown-6 ether (i.e. 1,4,7,10,13,16-... 

The L M in the complexes of lanthanide nitrates with TMSO decreases along the lanthanide series (264, 265). All these complexes contain both bidentate and mono-dentate nitrate groups (264), the monodentate nitrates giving way to bidentate nitrates as the cationic radius decreases. 

As one traverses through the lanthanide series, there is a reduction in the cation size as the atomic number increases. This results in small differences in the strength of interactions of the ligand with the lanthanide ions. These trends are reflected in the IR spectra of these complexes in a few cases. Cousins and Hart (203) have observed an increase in Pp Q with decreasing lanthanide ion radius for the complexes of TPPO with lanthanide nitrates. This observation has been attributed to an increase in the Ln—O bond strength with an increase in the atomic number of the lanthanide ion. 

Neutral extracting agents possessing oxygen-donor atoms (hard bases) in their structure easily coordinate trivalent lanthanide and actinide cations, but do not discriminate between the two families of elements, because the ion-dipole (or ion-induced dipole type) interactions mostly rely on the charge densities of the electron donor and acceptor atoms. As a result, the similar cation radii of some An(III) and Ln(III) and the constriction of the cation radius along the two series of /elements make An(III)/Ln(III) separation essentially impossible from nitric acid media. They can be separated, however, if soft-donor anions, such as thiocyanates, SCN-, are introduced in the feed (34, 35, 39, 77). 

Classical methods of separation [7] are (1) fractional crystallization, (2) precipitation and (3) thermal reactions. Fractional crystallization is an effective method for lanthanides at the lower end of the series, which differ in cation radius to a large extent. The separation of lanthanum as a double nitrate, La(N03)3-2NH4N03-4H20, from praseodymium and other trivalent lanthanide with prior removal of cerium as Ce4+ is quite a rapid process and is of commercial significance. Other examples are separation of yttrium earths as bromates, RE(Br03>9H20 and use of simple nitrates, sulfates and double sulfate and alkali metal rare earth ethylenediamine tetraacetate complex salts in fractional crystallization separation. 

With ethylenediamine complexes of the formula Ln(en)3X3 and Ln(en)4X3, where X = C1 , Br , NO, CIOJ have been characterized. IR data indicate that the tris and tetrakis complexes of the fighter lanthanides La-Sm, contain both ionic and coordinated nitrate groups. By contrast tetrakis complexes of heavier lanthanides, Eu-Yb contain ionic nitrate. This is possibly due to steric factors resulting from decreasing cationic radius that force the nitrate out of the coordination sphere of the lanthanides. A coordination number of 8 for tris complexes and a number of 9 for fighter lanthanide tetrakis complexes appears reasonable [234]. The thermodynamic parameters obtained show enthalpy stabilization for... 

There are four different phases of rare earth orthophosphate (RPO4), mostly depending on the cationic radius of rare earth element Monazite (monoclinic, dehydrate, for light lanthanides), xenotime (also typed as zircon, tetragonal, dehydrate or hydrate, for heavy lanthanides and Y +), rhabdophane (hexagonal, mostly hydrate, across the series), and... 

In terms of this model, both cation size and differences in the sizes of the various cations are important. In any state of oxidation, a lanthanide ion is comparatively large. Strong electrostatic attractions between such an ion and a ligand must, therefore, be limited. These attractions may be expected to increase in magnitude for a given oxidation state, as the cationic radius decreases, and for a given lanthanide element, as the cationic charge increases. Both of these variations are well established (36). 

A is the length of the hydrogen bond. Note that d(H ) = d(HH)/t ) = 0.28 A, the radius of the proton, is the cationic radius of H. Fig. 12.3 for Na and Cf ions shows a comparison of the covalent radii with the ionic radii in the crystal and in aqueous solutions. The results for many other ions (including Lanthanides) can be found in [7]. 

The rates of the isothermal dehydration of these salts were studied at 130, 150 and 170"C using TGA (Saito 1988). Linear plots of [1 — (1 — versus time (a is the degree of reaction) reflect a surface-controlled dehydration mechanism. The activation energies calculated from these data, which agreed with those reported earlier (Nathans and Wendlandt 1962), are inversely proportional to the cationic radius r. Such a relation is expected for the ionic interactions dominant in lanthanide complexes. The entropy of activation AS for these solid phase reactions varies from — 226 J K mol for La to — 123 J K mol for Tm. The variation in entropy is related to changes in the degree of rotational freedom of water molecules in the activated states as a result of lattice expansion. The linearity of the plot of A versus AS was interpreted as evidence for a common dehydration mechanism independent of the particular lanthanide cation present. 

The Raman spectra of solid ethylsulfate salts, [Ln(H 20)9] ( 8)3, showed a regular shift in v(Ln-O) frequency with cationic radius (Yamauchi et al. 1988). Kanno and Hiraishi (1980,1982,1988) and Yamauchi et al. (1988) have reported Raman studies of lanthanide chlorides and nitrates at room and at liquid nitrogen (in the glassy state)... 

Fig. 14. Plot of the log of the formation rate constant, log f, of the lanthanide oxalate complex as a function of the inverse cation radius.

Suzuki et al. (1986) measured the onset of precipitation of LnfOH), by optical detection. To avoid local concentration effects, the hydroxide ion was generated electrolytically. The pH values associated with initial precipitation in this study were substantially lower than those reported earlier (Moeller and Kremer 1945), presumably, due to differences in the sensitivity of detection. A plot of the pH of initial precipitation as a function of the radius of the lanthanide cations was an S-shaped... 

David and Fourest (1990) challenged the concept of a constant radius for the water molecule, rw- The electric field of the ions polarizes the water molecules adjacent to them and for multiply charged ions squeezes these molecules somewhat in the direction of the ions. The values of rw decrease according to these authors from 0.143 nm for the alkali metal cations down to 0.133nm for the divalent lanthanide cations. Therefore, using the mean value, rw = 0.138 nm, increases accordingly the uncertainty of the ionic radii in solution from 0.002 to 0.005 nm. 

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