Hydration light lanthanides

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 fig. 6 a clear discontinuity is seen in Ln OHj bond distances of curve A between Nd and Tb with an offset value of 0.045 A. This value agrees with the analysis of Sinha (1976) who calculated a 0.047 A decrease in the effective La radius per unit decrease in the coordination number. In the solid heptahydrate (La Pr) and hexa-hydrates (Nd-Lu), the offset value in the Ln OHj distances between the heavy and the light lanthanides is only 0.039 A. The smaller value was attributed to the differences in the structures of the heptahydrate and hexahydrales (the former form dimers with chloride bridging). 

The evidence presented in this section from visible spectral data supports a hydration number of nine for the light lanthanides. Although there is evidence for a lower hydration number for the heavy lanthanides, alternate explanations for the observed differences in spectral characteristics between the lighter and the heavier members of the series cannot be ruled out of consideration. 

The crystallographic ionic radii of the rare-earth elements in oxidation states +2 (CN = 6), +3 (CN = 6), and +4 (CN = 6) are presented in Table 18.1.3. The data provide a set of conventional size parameters for the calculation of hydration energies. It should be noted that in most lanthanide(III) complexes the Ln3+ center is surrounded by eight or more ligands, and that in aqueous solution the primary coordination sphere has eight and nine aqua ligands for light and heavy Ln3+ ions, respectively. The crystal radii of Ln3+ ions with CN = 8 are listed in Table 18.1.1. 

Inspection of the proposed nitration mechanism (Scheme 1) reveals that the mononitrate dipositive lanthanide species [Ln(H20)x(N03)](0Tf)2 (1) is the key intermediate. An independent preparation and characterisation of such a species enables possible indentification of 1 directly in situ in the reaction mixture. Additionally, spectroscopic examination of these salts may provide some evidence for our working model. We have developed a novel preparation of these mixed salts by simple metathesis of lanthanide chlorides with the requisite quantities of silver nitrate and silver triflate in water (Scheme 3).17 The resulting hydrated salts were white or lightly coloured (pink, green or yellow) solids which were found to be stable indefinitely at room temperature in the solid state. 

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. 

Lanthanide p-diketonates are amongst the best smdied rare-earth luminescent complexes [58]. They are brightly luminescent and volatile so that incorporation into various electroluminescent materials is simple. Moreover their photophysical properties are easily tuned by a judicious choice of ancillary ligands. Indeed, conventional synthesis usually yields bis(hydrated) lanthanide tris(P-diketonates), but the two solvent molecules can be substituted by either a fourth diketonate anion or a donor ligand with adequate functionalisation as to provide convenient light harvesting and subsequent energy transfer onto the metal ion. It is noteworthy that not only visible but also near-infrared luminescence [59,60] is efficiently sensitised in lanthanide p-diketonates. In the case of Eu , some ternary complexes have quantum yields up to 85% [8] and the main asset of their luminescent properties is an emission essentially concentrated in the hypersensitive Dq transition... 

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