Lanthanides trivalency

This spectrum has been compared with the BIS measurement on Nd metaP i.e. of the homologous lanthanide. Trivalent Nd has a localized 4f initial state configuration. For U, a 5 f or a 5 f initial state configuration are usually assumed, with a tetravalent or a trivalent core respectively. While in Nd the 4f and 4f multiplet states, as evaluated in an atomic-like Russell-Saunders scheme, can be well recognized in the XPS/BIS combined results, and are well separated from a (weak) d-emission at the Fermi edge, in U the occupied states and the empty states spectra join in a continuous band at Ep. Therefore, only the symmetry of 5 f states, given by the position of the main peaks in the joint spectrum, can be recognized with certainty. 

Fig. 10 The product function /moi T vs. kT/X for lanthanide trivalent ions left - less than half-filled shells, right - more than half filled shells (X < 0) numbers correspond to the/ configuration...

The grafting of CMPO moieties on the narrow rim affords a strong decrease of extracting ability toward lanthanides, trivalent actinides, and tetravalent plutonium from acidic solutions. The distribution ratios for the different calixarenes in NPHE are low, except for CPn3 for which the number of carbon atoms in the spacer is four, but even for this compound, the distribution ratios are lower than those obtained with their wide-rim counterparts (Figure 4.11). 

Marmier, N. et al.. Etude experimentale et modelisation de la sorption des ions lanthanide trivalents sur Thematite, C. R. Acad. Sci. Paris, 317, 311, 1993. 

The one review which treats lanthanide/trivalent actinide separation is that of Weaver (1974). Weaver s review is an excellent, if somewhat dated, source for a comprehensive discussion of solvent extraction separations of the lanthanides and trivalent actinides. Weaver discusses many of the historical aspects of lanthanide/ actinide separation, and considers both the successes and failures in the separation of trivalent lanthanides and actinides. 

There are very few reported values of the higher monomeric stability constants of gadolinium. The values that have been reported are listed in Table 8.37. There are only four reported values for the stability of Gd(OH)2 and three values for the stability of Gd(OH)g(aq). None of the data appear consistent with the stability constant selected for GdOH " in this review. In all cases, the stability constant would lead to a stepwise stability where either log K2 or log K is greater than or equal to log K. This behaviour is considered unlikely for the lanthanide trivalent metal ions, and consequently, none of the values are retained by this... 

First, the use of water limits the choice of Lewis-acid catalysts. The most active Lewis acids such as BFj, TiQ4 and AlClj react violently with water and cannot be used However, bivalent transition metal ions and trivalent lanthanide ions have proven to be active catalysts in aqueous solution for other organic reactions and are anticipated to be good candidates for the catalysis of aqueous Diels-Alder reactions. 

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 arc and spark spectra of the individual lanthanides are exceedingly complex. Thousands of emission lines are observed. For the trivalent rare-earth ions in soUds, the absorption spectra are much better understood. However, the crystal fields of the neighboring atoms remove the degeneracy of some states and several levels exist where only one did before. Many of these crystal field levels exist very close to a base level. As the soUd is heated, a number of the lower levels become occupied. Some physical properties of rare-earth metals are thus very sensitive to temperature (7). 

Chemical Properties. Although the chemical properties of the trivalent lanthanides are quite similar, some differences occur as a consequence of the lanthanide contraction (see Table 3). The chemical properties of yttrium are very similar too, on account of its external electronic stmcture and ionic radius. Yttrium and the lanthanides are typical hard acids, and bind preferably with hard bases such as oxygen-based ligands. Nevertheless they also bind with soft bases, typicaUy sulfur and nitrogen-based ligands in the absence of hard base ligands. 

In aqueous solutions, trivalent lanthanides ate very stable whereas only a limited number of lanthanides exhibit a stable divalent or tetravalent state. Practically, only Ce and Eu " exist in aqueous solutions. The properties of these cations ate very different from the properties of the trivalent lanthanides. For example, Ce" " is mote acidic and cetium(IV) hydroxide precipitates at pH 1. Eu " is less acidic and eutopium(II) hydroxide does not precipitate at pH 7—8.5, whereas trivalent lanthanide hydroxides do. Some industrial separations ate based on these phenomena. 

In the area of superconductivity, tetravalent thorium is used to replace trivalent lanthanides in n-ty e doped superconductors, R2 Th Cu0 g, where R = Pr, Nd, or Sm, producing a higher T superconductor. Thorium also forms alloys with a wide variety of metals. In particular, thorium is used in magnesium alloys to extend the temperature range over which stmctural properties are exhibited that are useful for the aircraft industry. More detailed discussions on thorium alloys are available (8,19). 

Ce(IV) extracts more readily iato organic solvents than do the trivalent Ln(III) ions providing a route to 99% and higher purity cerium compounds. Any Ce(III) content of mixed lanthanide aqueous systems can be oxidi2ed to Ce(IV) and the resultiag solutioa, eg, of nitrates, contacted with an organic extractant such as tributyl phosphate dissolved in kerosene. The Ce(IV) preferentially transfers into the organic phase. In a separate step the cerium can be recovered by reduction to Ce(III) followed by extraction back into the aqueous phase. Cerium is then precipitated and calcined to produce the oxide. 

Cerium is strongly electropositive having a low ionization potential for the removal of the three most weakly bound electrons. The trivalent cerous ion [18923-26-7] Ce ", apart from its possible oxidation to Ce(IV), closely resembles, the other trivalent lanthanides in behavior. 

Cerous salts in general are colorless because Ce " has no absorption bands in the visible. Trivalent cerium, however, is one of the few lanthanide ions in which parity-allowed transitions between 4f and Sd configurations can take place and as a result Ce(III) compounds absorb in the ultraviolet region just outside the visible. 

Figure 30.3 Variation with atomic number of some properties of La and the lanthanides A, the third ionization energy (fa) B, the sum of the first three ionization energies ( /) C, the enthalpy of hydration of the gaseous trivalent ions (—A/Zhyd)- The irregular variations in I3 and /, which refer to redox processes, should be contrasted with the smooth variation in A/Zhyd, for which the 4f configuration of Ln is unaltered.

Harrowfield et al. [37-39] have described the structures of several dimethyl sulfoxide adducts of homo bimetallic complexes of rare earth metal cations with p-/e rt-butyl calix[8]arene and i /i-ferrocene derivatives of bridged calix[4]arenes. Ludwing et al. [40] described the solvent extraction behavior of three calixarene-type cyclophanes toward trivalent lanthanides La (Ln = La, Nd, Eu, Er, and Yb). By using p-tert-huty ca-lix[6Jarene hexacarboxylic acid, the lanthanides were extracted from the aqueous phase at pH 2-3.5. The ex-tractability is Nb, Eu > La > Er > Yb. 

In what follows we briefly review some of the previous attempts to analyze the available spectra of plutonium (6). In addition, we estimate energy level parameters that identify at least the gross features characteristic of the spectra of plutonium in various valence states in the lower energy range where in most cases, several isolated absorption bands can be discerned. The method used was based on our interpretation of trivalent actinide and lanthanide spectra, and the generalized model referred to earlier in the discussion of free-ion spectra. 

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