Ligands tetravalent lanthanide compounds

The chemistry of rare earths is often discussed only in terms of the trivalent ions and indeed, contrary to the actinides, the oxidation states encountered in lanthanide compounds in the solid state and especially in solution are few in number. Standard electrode potentials M(II-III) and M(III-IV) indicate that, besides the trivalent rare earth ions, only Eu (-0.35 V), Yb + ( — 1.15 V), Sm + ( — 1.55 V) and Ce (+1.74 V) are sufficiently stable to exist in aqueous solutions (Nugent, 1975). It has long been known that alkaline conditions and many complexing anions such as nitrate, phosphate and sulfate stabilize Ce(IV) (Jorgensen, 1979) and recently it has been shown that large complex-forming ligands such as heteropolyanions also stabilize to some extent tetravalent praseodymium and terbium (Spitsyn, 1977). 

Fig. 12. Demonstration of typical chemical shifts in lanthanide Lm absorption. The energy calibration of the spectra recorded from the Pr absorption in the compounds is accurate within 0.2 eV with respect to o fixed at the intersection point of the high-energy absorption with the absorption line in (dhcp) Pr metal (dashed-dotted line). The intersection point shifts to higher energies with decreasing metallic character of the compounds. The maxima of the prominent main lines, however, remain unshifted, just as the onsets of the lines. PrCu crystallizes in orthorhombic FeB structure (a = 7.343 A, 6 = 4.584 A, c = 5.604 A). The semi-metals PrSb, PrBi have fee (NaCl) structure a = 6.366 and 6.463 A, respectively). Pr Oi] is a nonstoichiometric modification of nominally tetravalent Pr02 (fluorite type, Cap2). The bar diagram indicates ligand-field-split absorption lines for both, the tri- and tetravalent valence states in Pr Oij (cf. section 14).

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