Trivalent hydrated actinides

A guiding principle for the solvent extraction chemist is to produce an uncharged species that has its maximum coordination number satisfied by lipophilic substances (reactants). Eor trivalent lanthanides and actinides (Ln and An, respectively), the thermodynamic data suggest a model in which addition of one molecule of TBP displaces more than one hydrate molecule ... 

Tits, J., Stumpf, Th., Rabung, Th., Wieland, E. Fanghanel, Th. 2003. Uptake of trivalent actinides (Cm(III)) and lanthanides (Eu(III)) by Calcium silicate hydrates a wet chemistry and time-resolved laser fluorescence spectroscopy (TRLFS) study. Environmental Science and Technology, 37, 3568-3573. 

An electrostatic hydration model has been applied to the trivalent lanthanide and actinide ions in order to predict the standard free energy (AG°) and enthalpy (AHt) of hydration for these series. Assuming crystallographic and gas-phase radii for Bk(III) to be 0.096 and 0.1534 nm, respectively, and using 6.1 as the primary hydration number, AG298 was calculated to be -3357 kJ/mol, and A/Z298 was calculated to be -3503 kJ/mol (187). 

Hydration and Hydrolysis. The various oxidation states of plutonium form strong ion-dipole bonds with water to become strongly hydrated in aqueous solution. To a first approximation, we can expect the hydration numbers of the first coordination sphere to be the same as the most probable coordination numbers suggested in the preceeding section. This means values of 8 or 9 for Pu(lll), 7 or 8 for Pu(Vl), and, perhaps, 4 for PuOj and 6 for PuOj. However, the polarization of the water dipoles of the primary hydration layer leads to attraction of additional waters of hydration. Estimates of the total number of waters of hydration for trivalent lanthanides and actinides have been given as 12 - 15 model of a small number of... 

There is not much information on the hydration of these trivalent cations. Estimates made on the basis of the calculated entropies of hydration suggest a hydration number of about 7 (5, 6a). But an estimate made by extrapolation of data for electrophoretic transport measurements on the ions of some heavier actinides indicate a hydration number of about 12.7, with nine water molecules in the first coordination sphere (66),... 

The trivalent transplutonium halides have been extensively studied. Several reviews deal specifically with actinide halides. " In aqueous solution the mono- and bis-complexes have been characterized, with the formation of the latter decreasing down the halide series. For example, AmF and AmF2" have been studied, but only a very weak monochloride complex AmCP has been reported. These species are reported to have coordination numbers as high as 11, although recent EXAFS studies show that the hydration number decreases with increasing halide concentration (and ionic strength) at concentrations below which the halo complexes form. These data suggest the coordination numbers of the mixed aquo halo complexes are probably seven to ten. 

Fig. 55. Variation of the hydration enthalpies (AHf,) and the entropies (ASj,) for the trivalent and tetravalent actinides with their L-values (58). Reproduced with the kind permission of Helvetica Chimica Acta.

Mochizuki and Okamoto applied the Dirac program for the estimation of stabilities of trivalent actinide elements and water or ammine complexes (Mochizuki and Okamoto 2002). Mochizuki and Tatewaki (2002) also carried out the electronic structure calculation on the hexa-hydrated ions of curium and gadolinium. They used the Dirac program and also predicted the fluorescence transition energy using the Complete Open-Shell Configuration Interaction (COSCI) method. Even the hexa-hydrate curium ion needs 2,108 basis functions for the fully relativistic four-component calculation. 

The other characteristic of the trivalent lanthanide and actinide series that can be exploited in separations is the decrease in ionic radius which occurs with increasing atomic number. This results in increased strength of cation-anion interactions and ion-dipole, ion-induced dipole type interactions. The expected increase in ion-dipole interactions across the series implies that the heavy members of both series should bind solute (and suitable solvent) molecules more tightly than the light members. For certain ion exchange separations, it is thus appropriate to expect elution trends to correlate with the hydrated cation radius rather than the simple cation radius. 

This chapter discusses recent developments in solvation dynamics and thermodynamics of f-element cations and the hydrol5ftic tendencies of these ions in aqueous media. Direct comparisons are made for the trivalent cations of the lanthanides and actinides while the data for the other oxidation states of the actinides are reviewed to assess the effects of charge density and structure on hydration and hydrolysis. Since the lanthanides were reviewed recently, major attention is given to the actinides and the similarities and differences in their behavior with that of the lanthanides. 

Hydration numbers and hydration radii of trivalent actinide and lanthanide ions obtained by electrophoresis and diffusion measurements (Lundqvist et al. 1981, Fourest et al. 1984, David 1986). 

Assuming that the interaction between the primary hydration sphere and the secondary water molecules of hydration are strongly electrostatic for both f-element series, Fourest et al. (1984) were able to estimate the inner-sphere hydration numbers of the trivalent actinide ions by interpolation using the values for the lanthanide elements (Habenschuss and Spedding 1979a, b, 1980). The interpolated values are listed in the last column of table 3 and shown in fig. 3. 

Fig. 3. S-shaped variation of the primary hydration number, N, for some trivalent lanthanides and actinides, versus (coordination number is eight) ( ) Values given by Haben-schussand Spedding(1979a,b, 1980) ( ) Interpolated values.

Studies of the complex-ion chemistry of Es have been made in conjunction with measurements of the stability constants of other trivalent actinides. A summary of the known stability constants for einsteinium complexes is shown in Table 12.5. With the possible exception of the two lower thiocyanate complexes, chloride is the only outer-sphere complex in which water of hydration lies between the ligand and einsteinium ioa The remaining complexes are believed to be inner-sphere as inferred from the increase of a given stability constant with increase in atomic number and from the enthalpy and entropy of formation of the complex. 

Oxalates of the trivalent actinides are important in separation chemistry, and their structures have received some attention. X-ray powder diffraction studies of the oxalates of Pu [439] and Am [440] have shown them to be isomorphous with the lanthanide analogs, while the oxalate of Cm that was made did not give useful diffraction patterns [441]. All of these oxalates have the nominal composition M2(C204)3-10H20 (M = lanthanide or actinide element), but the degree of hydration has been shown to vary somewhat depending on the method of... 

Stumpf T, Fanghanel T and Grenthe 1 2002 Complexation of trivalent actinide and lanthanide ions by glycolic acid a trlfs study. J. Chem. Soc., Dalton Trans, pp. 3799-3804. Lindqvist-Reis P, Klenze R, Schubert G and Fanghl T 2005 Hydration of cm3+ in aqueous solution from 20 to 200 c. a time-resolved laser fluorescence spectroscopy study. The Journal of Physical Chemistry BimO), 3077-3083. PMID 16851323. 

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