Cations, trivalent lanthanide

Unlike alkali-metal or alkaline-earth-metal cations, trivalent lanthanides hydrolyze significantly at this pH. 

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. 

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. 

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. 

Trivalent lanthanide cations have luminescent properties which are used in a number of applications. The luminescence of the lanthanide ions is unique in that it is long-lasting (up to more than a millisecond) and consists of very sharp bands. Lanthanide emission, in contrast to other long-lived emission processes, is not particularly sensitive to quenching by oxygen because the 4f electrons found within the inner electron core... 

Another area where titration calorimetry has found intensive application, and where the importance of heat flow versus isoperibol calorimetry has been growing, is the energetics of metal-ligand complexation. Morss, Nash, and Ensor [225], for example, used potenciometric titrations and heat flow isothermal titration calorimetry to study the complexation of UO "1" and trivalent lanthanide cations by tetrahydrofuran-2,3,4,5-tetracarboxylic acid (THFTCA), in aqueous solution. Their general goal was to investigate the potential application of THFTCA for actinide and lanthanide separation, and nuclear fuels processing. The obtained results (table 11.1) indicated that the 1 1 complexes formed in the reaction (M = La, Nd, Eu, Dy, andTm)... 

Neutral N-derivatized octadentate ligands based on cyclen (1,4,7,10-tetraazacy-clododecane) form tripositive cationic complexes with the trivalent lanthanides [116-124]. The N-substituted tetraamide derivatives have proven useful in understanding the relationship between the solution structure of the Ln3+ complex and its water exchange rate, a critical issue in attaining optimal relaxation efficiency of CA s [125-131]. 

Jensen, M.P., Bond, A.H. 2002. Influence of aggregation on the extraction of trivalent lanthanide and actinide cations by purified Cyanex 272, Cyanex 301, and Cyanex 302. Radiochim. Acta 90 (4) 205-209. 

Neutral extracting agents possessing oxygen-donor atoms (hard bases) in their structure easily coordinate trivalent lanthanide and actinide cations, but do not discriminate between the two families of elements, because the ion-dipole (or ion-induced dipole type) interactions mostly rely on the charge densities of the electron donor and acceptor atoms. As a result, the similar cation radii of some An(III) and Ln(III) and the constriction of the cation radius along the two series of /elements make An(III)/Ln(III) separation essentially impossible from nitric acid media. They can be separated, however, if soft-donor anions, such as thiocyanates, SCN-, are introduced in the feed (34, 35, 39, 77). 

There are various types of organic proton exchangers (34, 35, 38). Diesters of phosphoric acid, (RO)2P = 0(0H), phosphonic acids, R(RO)P = 0(0H), and phos-phinic acids, R2P = 0(0H), where R represents linear or branched alkyl or phenyl substituents, are the most common cation exchangers developed in liquid-liquid extraction for the extraction of trivalent 4/and 5/elements. They were initially developed for the American TALSPEAK and the Japanese DIDPA processes and have recently been introduced in the French DIAMEX-SANEX process. As for previously described NOPCs, these organophosphorus acids present oxygen-donor atoms (hard bases) in their structures and therefore will easily coordinate trivalent lanthanide and actinide cations, but they will not allow complete discrimination of the two families of elements. However, contrary to previously described neutral organophosphorus... 

The cadmium(II) complex corresponding to 9 (M = Cd n = 2) was the first texaphyrin made [6], This aromatic expanded porphyrin was found to differ substantially from various porphyrin complexes and it was noted that its spectral and photophysical properties were such that it might prove useful as a PDT agent. However, it was also appreciated that the poor aqueous solubility and inherent toxicity of this particular metal complex would likely preclude its use in vivo [29-31], Nonetheless, the coordination chemistry of texaphyrins such as 9 was soon generalized to allow for the coordination of late first row transition metal (Mn(II), Co(II), Ni(II), Zn (II), Fe(III)) and trivalent lanthanide cations [26], This, in turn, opened up several possibilities for rational drag development. For instance, the Mn(II) texaphyrin complex was found to act as a peroxynitrite decomposition catalyst [32] and is being studied currently for possible use in treating amyotrophic lateral sclerosis. This work, which is outside the scope of this review, has recently been summarized by Crow [33],... 

Alkaline Earth Oxides. If a trivalent lanthanide is substituted for Ca + in CaO (5) or a Cr3+ is substituted for a Mg2+ in MgO (6), the extra positive charge is compensated by a cation vacancy as described earlier. The vacancy can be in a near neighbor position or it can be very distant from the probe ion (7-10). If another ion like Na+ is in the cation site, it can serve as a charge compensation for the probe ion either as a nearby ion or distantly. This case is coupled ion substitution that commonly affects the partitioning of ions into different mineral phases (11). One can envision many other ions that could also affect the local environment or the probe ion. 

Classical methods of separation [7] are (1) fractional crystallization, (2) precipitation and (3) thermal reactions. Fractional crystallization is an effective method for lanthanides at the lower end of the series, which differ in cation radius to a large extent. The separation of lanthanum as a double nitrate, La(N03)3-2NH4N03-4H20, from praseodymium and other trivalent lanthanide with prior removal of cerium as Ce4+ is quite a rapid process and is of commercial significance. Other examples are separation of yttrium earths as bromates, RE(Br03>9H20 and use of simple nitrates, sulfates and double sulfate and alkali metal rare earth ethylenediamine tetraacetate complex salts in fractional crystallization separation. 

It is not surprising that perovskite oxides continue to attract unusual levels of attention. In this category, we include the Ruddlesden Popper phases of compositions (A0)(AT03) where A is a large cation, either a divalent alkaline earth or a trivalent lanthanide, and T is a transition element. The index, n, enumerates the number of perovskite ATO3 units intergrown with rock salt AO layers. 

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