Lanthanide lighter

Fra.ctiona.1 Precipituition. A preliminary enrichment of certain lanthanides can be carried out by selective precipitation of the hydroxides or double salts. The lighter lanthanides (La, Ce, Pr, Nd, Sm) do not easily form soluble double sulfates, whereas those of the heavier lanthanides (Ho, Er, Tm, Yb, Lu) and yttrium are soluble. Generally, the use of this method has been confined to cmde separation of the rare-earth mixture into three groups light, medium, and heavy. 

There are few principal lanthanide deposits, and there are no minerals that are sources for cerium alone. All the lighter lanthanides occur together in any potential deposit, and processes separating the lanthanides are necessary to obtain pure cerium products. 

Scandium is very widely but thinly distributed and its only rich mineral is the rare thortveitite, Sc2Si20v (p. 348), found in Norway, but since scandium has only small-scale commercial use, and can be obtained as a byproduct in the extraction of other materials, this is not a critical problem. Yttrium and lanthanum are invariably associated with lanthanide elements, the former (Y) with the heavier or Yttrium group lanthanides in minerals such as xenotime, M "P04 and gadolinite, M M SijOio (M = Fe, Be), and the latter (La) with the lighter or cerium group lanthanides in minerals such as monazite, M P04 and bastnaesite, M C03F. This association of similar metals is a reflection of their ionic radii. While La is similar in size to the early lanthanides which immediately follow it in the periodic table, Y , because of the steady fall in ionic radius along the lanthanide series (p. 1234), is more akin to the later lanthanides. 

The classical methods used to separate the lanthanides from aqueous solutions depended on (i) differences in basicity, the less-basic hydroxides of the heavy lanthanides precipitating before those of the lighter ones on gradual addition of alkali (ii) differences in solubility of salts such as oxalates, double sulfates, and double nitrates and (iii) conversion, if possible, to an oxidation state other than -1-3, e g. Ce(IV), Eu(II). This latter process provided the cleanest method but was only occasionally applicable. Methods (i) and (ii) required much repetition to be effective, and fractional recrystallizations were sometimes repeated thousands of times. (In 1911 the American C. James performed 15 000 recrystallizations in order to obtain pure thulium bromate). 

By the time Ho is reached the Ln radius has been sufficiently reduced to be almost identical with that of Y which is why this much lighter element is invariably associated with the heavier lanthanides. 

However, solubility, depending as it does on the rather small difference between solvation energy and lattice energy (both large quantities which themselves increase as cation size decreases) and on entropy effects, cannot be simply related to cation radius. No consistent trends are apparent in aqueous, or for that matter nonaqueous, solutions but an empirical distinction can often be made between the lighter cerium lanthanides and the heavier yttrium lanthanides. Thus oxalates, double sulfates and double nitrates of the former are rather less soluble and basic nitrates more soluble than those of the latter. The differences are by no means sharp, but classical separation procedures depended on them. 

The structural chemistry of the actinides is often similar to that of lighter transition metals, such as Zr and Hf, and to that of the lanthanides however, the diffuse nature of the 5/ orbitals leads to some differences and specifically to interesting magnetic and electrical properties. The actinide sulfides are generally isostructural with the selenides, but not with the analogous tellurides. The binary chalcogenides of uranium and thorium have been discussed in detail [66], but the structural... 

For the lighter elements rei is much less than unity (e.g., rei = 0.15 for Sc) and the relativistic corrections are small. However, these corrections become much larger for heavier elements (e.g., ,ei = 0.58 for Hg, about four times larger than for Sc) and the perturbative correction procedure breaks down completely as rei approaches unity. Thus, while relativistic corrections are largely ignorable for the first transition series, these corrections become of dominant chemical importance in later series, particularly after filling of the lanthanide f shell. 

Complexes of the lanthanides with a few cyclic amides are known. Miller and Madan have reported the complexes of 7-butyrolactam with lanthanide nitrates (60) and perchlorates (61). Complexes of lanthanide perchlorates and lighter lanthanide nitrates with BuL have a L M of 8 1. However, complexes of heavier lanthanide nitrates have a L M of only 3 1. By changing the solvent used for the crystallization of the abovementioned complexes, complexes of the formula [La(BuL)4(N03)3] and [Gd(BuL)3(N03)3] could be prepared (60). Complexes of NMBuL (61, 62) and CLM (63-66) have also been reported. 

Probably, the first series of lanthanide complexes with neutral oxygen donor ligands is that of AP with the lanthanide nitrates. In 1913, Kolb (79) reported tris-AP complexes with lighter lanthanide nitrates and tetrakis-AP complexes with heavier lanthanide nitrates. Subsequently, complexes of lanthanide nitrates with AP which have a L M of 6 1 and 3 1 have also been prepared (80-82). Bhandary et al. (83) have recently shown through an X-ray crystal and molecular structure study of Nd(AP)3(N03)3 that all the nitrates are bidentate and hence the coordination number for Nd(III) is nine in this complex. Complexes of AP with lanthanide perchlorates (81, 84), iodides (81, 85), and isothiocyanates (66, 86, 87) are known. While the perchlorates and iodides in the respective complexes remain ionic, two of the isothiocyanates are coordinated in the corresponding complexes of AP with lanthanide isothiocyanates. 

Castellani Bisi (98) has synthesized complexes of lanthanide perchlorates with DMP which have a L M of 8 1 for the lighter lanthanide and 7 1 for the heavier lanthanide complexes. These complexes were prepared by reacting the respective metal salts with an excess of the ligand. When the complexes were prepared under conditions of lower concentrations of the ligand, complexes of DMP with a L M of 6 1 were obtained. The perchlorate groups in all three groups of complexes are ionic. 

Vicentini and Dunstan (227) have obtained tetrakis-DDPA complexes with lanthanide perchlorates in which the perchlorate groups are shown to be coordinated to the metal ion. DDPA also yields complexes with lanthanide isothiocyanates (228) and nitrates (229). All the anions in these complexes are coordinated. DPPM behaves more or less like DDPA which is reflected in the stoichiometry of the complexes of DPPM with lanthanide perchlorates (230), nitrates, and isothiocyanates (231). Hexakis-DMMP complexes of lanthanide perchlorates were recently reported by Mikulski et al. (210). One of the perchlorate groups is coordinated to the metal ion in the lighter lanthanide complexes, and in the heavier ones all the perchlorate groups are ionic. 

Sylvanovich and Madan (234) have isolated the complexes of OMPA with lanthanide nitrates. With the lighter lanthanide nitrates, bis-OMPA complexes were obtained and the heavier lanthanide nitrates yielded complexes of the type Ln2(OMPA)3-(N03)6. In the latter complexes both bridging and chelating ligands are present. Complexes of OMPA with lanthanide perchlorates are also known (235). Airoldi et al. 

Since 1972, complexes of lanthanides with cyclic sulfoxides have received considerable attention. Zinner and Vicentini (261) have reported the complexes of lanthanide perchlorates with TMSO. The L M in these complexes decreases along the lanthanide series. But in the case of complexes of lanthanide chlorides with TMSO, the L M increases from 2 1 for the lighter lanthanides to 3 1 for the heavier lanthanides (262). It has been suggested that these complexes, especially the bis-TMSO complexes, contain bridging chloride ions. Tetrakis-TMSO complexes with lanthanide isothiocyanates have also been reported (263). 

The prices of lanthanide elements are somewhat reasonable and are less than gold per kilogram. (Gold is about 1,800 per kg.) Cesium (Ce), which is relatively common, is often alloyed with La, Nd, and Pr and iron to form misch metal. This alloy has several uses based on its unique ability to spark when scratched. The most common use is as flints for cigarette lighters. 

The lanthanide trichlorides have three different crystal structures. The trichlorides of the lighter lanthanides (La—Gd) have the UCI3 type (P63/OT) structure, while the trichlorides of the lanthanides heavier than terbium have the YCI3 type (C2/ot) structure 67, 174, 175). Terbium trichloride and a second form of DyCls have a third structure (PuBr3 type, Cmcni) 176). 

These two long rows of elements are traditionally moved to the base of the chart so the more important, lighter elements may be closer together for clarity. These two rows of metals each reflect the progressive addition of 14 electrons into an /-type subshell. The lanthanides occur in only trace amounts in nature and are often called rare earths. All of the actinides have large, unstable nuclei that undergo spontaneous radioactive decay. 

Ion exchange techniques have been widely applied in the separation process. In the large-scale ammonium thiocyanate process, the metal is retained on strong base anion exchanger thus, separating it from the lighter lanthanide elements which are not strongly absorbed on the resin. 

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