Praseodymium tetravalent

Although rare-earth ions are mosdy trivalent, lanthanides can exist in the divalent or tetravalent state when the electronic configuration is close to the stable empty, half-fUed, or completely fiUed sheUs. Thus samarium, europium, thuUum, and ytterbium can exist as divalent cations in certain environments. On the other hand, tetravalent cerium, praseodymium, and terbium are found, even as oxides where trivalent and tetravalent states often coexist. The stabili2ation of the different valence states for particular rare earths is sometimes used for separation from the other trivalent lanthanides. The chemicals properties of the di- and tetravalent ions are significantly different. 

The lanthanides, unlike the transition metals and the actinides, tend not to form compounds over a range of oxidation states. The +3 oxidation state is characteristic of all of the lanthanides, and the oxide fluorides of formula LnOF (Ln = lanthanide metal) are well known. The less stable oxidation states of + 2 and + 4 are known, but the latter is represented only by the dioxides and tetrafluorides of cerium, praseodymium, and terbium, and no tetravalent oxide fluorides have been reported. 

Despite Brauner s belief in the validity of the Mendeleev methodology, he also had to admit that he had not yet succeeded in resolving the rare-earth crisis. Thus Brauner wrote in 1901 with reference to praseodymium that its maximum valency was tetravalent, like that of cerium but that no place had been found in the periodic table for an element possessing the physical and chemical properties of praseodymium and its compounds (Brauner, 1901b). He also admitted that the difficulties of finding a place for neodymium in the periodic table were even greater than in the case of praseodymium. 

Preparation, Structure and Spectra of Some Tetravalent Praseodymium Compounds... 

Figure 1. Structure of first coordination sphere of fluorides around tetravalent praseodymium ion...

Because the tripositive ions are the most stable for all the rare earth elements in almost all compounds, the thermochemistry of the solid (crystalline) rare earth sesquioxides dominates this chapter. Some rare earths have divalent or tetravalent states, so the chemistry of solid monoxides and dioxides are included. There are also many nonstoichiometric binary oxides of cerium, praseodymium, and terbium. As much as possible, the thermochemistry of these nonstoichiometric binary oxides is included. The stability, phase diagrams, and structures of ternary and polynary... 

Few lanthanide elements may be oxidized to the tetravalent state and stabilized, almost exclusively, in fluorides and oxides. These are the elements cerium, terbium, praseodymium, dysprosium, neodymium, holmium, for example all in the ternary fluorides CS3RF7 (Hoppe and Roedder 1961). There are also hints at pentavalent praseodymium, CsPrFe would be the example (Hoppe 1980). 

Asprey, L.B., J.S. Colman and M.J. Reisfeld, 1967, Preparation, Structure and Spectra of some Tetravalent Praseodymium Compounds, in Gould, R.F., ed., Advances in Chemistry Series No. 71 (American Chemical Society, Washington), pp. 122-126. 

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). 

The thermal dissociation of praseodymium carbonate in an O2 atmosphere yields an intermediate tetravalent carbonate. This decomposes at much lower temperature (295-310°C) than tetravalent cerium carbonate (Pajakoflf, 1968). 

Cerium is the most abundant element of the rare earths. On average the Earth s crast contains 66 ppm of cerium (=66 g per ton), a value that is very comparable with the abundance of copper (68 ppm) (Emsley, 1991). Eew people know that there are on Earth larger resources of cerium than of other more popular elements like cobalt (29 ppm), lead (13 ppm), tin (2.1 ppm), silver (0.08 ppm) or gold (0.004 ppm). A special property of cerium is that it has a stable tetravalent oxidation state besides the trivalent state which is so common for the rare earths. Although the tetravalent oxidation state is also known for solid state compounds of praseodymium and terbium, cerium is the only rare-earth element that has a stable tetravalent oxidation state in solution. Many of the applications of cerium are based on the one-electron Ce +/Ce + redox couple. 

Fractional crystallization is most effective at the lower (lanthanum) end of the lanthanide series, where differences in cationic radius are greatest. The separation of lanthanum as the double ammonium nitrate, La(N0j)j-2NH4N03-4H20, from praseodymium and other tervalent lanthanons (after prior removal of cerium in its tetravalent state) is sufficiently rapid and effective to be of commercial significance, but no other procedure of this type is of technical importance today. 

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