Neodymium hydrates

It has always been assumed that the hydration numbers for the lanthanides are higher than six, probably between 8 and 10, in analogy with the presence of enneaaquo ion [M(OH2)9 +] in neodymium bromate and ethylsulphate (see later, p. 121). Lanthanide hydration numbers have not been rigorously established, but some attempts have been made to study the problem by NMR technique (13—15). It is rather unfortunate that only low value for the hydration numbers ( 6) have been obtained, except for Er(III) and Yb(III) ions (756), where the hydration number is seven. 

The anhydrous oxide absorbs moisture from the air at ambient temperatures forming hydrated oxide. The oxide also absorbs carbon dioxide from air, forming neodymium carbonate. 

Na2iHi36Moi54053oP7 -300H2O, Molybdate, polyoxo-, wheel-shaped cluster with hypo-phosphite ion, hydrate, 34 199 Nd4Cl8N605gCi8H66, Neodymium(III),... 

The single crystal X-ray structure of hydrated neodymium hypophosphate, NdHP206-4H20, has been determined.239 The Nd ion is eight-coordinated to four H20 and two bidentate hypophosphate anions. The details of interatomic distances and bond angles were compared with other members of this isostructural series, which extends at least from Nd to Er.240... 

A monomeric (and anhydrous) neodecanoate (= NDH) was obtained from the ligand exchange reaction between hydrated neodymium(III) acetate Nd(0Ac)3(H20)6 and neodecanoic acid (Scheme 14). The coordination sphere of the large neodymium cation is saturated by an additional molecule of neodecanoic acid, as evidenced by MALDI-TOF mass spec-... 

Scheme 14 Synthesis of oligomeric (ND) and monomeric neodymium(III) neodecanoates (NDH) from hydrated neodymium chlorides and acetates [124]...

The only complexes of lanthanum or cerium to be described are [La(terpy)3][C104]3 175) and Ce(terpy)Cl3 H20 411). The lanthanum compound is a 1 3 electrolyte in MeCN or MeN02, and is almost certainly a nine-coordinate mononuclear species the structure of the cerium compound is not known with any certainty. A number of workers have reported hydrated 1 1 complexes of terpy with praseodymium chloride 376,411,438), and the complex PrCl3(terpy)-8H20 has been structurally characterized 376). The metal is in nine-coordinate monocapped square-antiprismatic [Pr(terpy)Cl(H20)5] cations (Fig. 24). Complexes with a 1 1 stoichiometry have also been described for neodymium 33, 409, 411, 413, 417), samarium 33, 411, 412), europium 33, 316, 411, 414, 417), gadolinium 33, 411), terbium 316, 410, 414), dysprosium 33, 410, 412), holmium 33, 410), erbium 33, 410, 417), thulium 410, 412), and ytterbium 410). The 1 2 stoichiometry has only been observed with the later lanthanides, europium 33, 411, 414), gadolinium, dysprosium, and erbium 33). 

Hydrates of rare earth chlorides also have two different crystal systems a triclinic system for lanthanum, cerium, and praseodymium, as well as a monoclinic system for neodymium to lutetium and yttrium. CeCl3-7H20, as an example of the former system, is different from the above infinite polymer as two cerium atoms are connected by two [i2-bridges to form a dimer. The formula for this dimer is [(H20)7Ce([i2-Cl)2Ce(H20)7]Cl4 as shown in Figure 1.18. Therefore, the coordination number of cerium is nine and the polyhedron takes on a destroyed mono-capped square antiprism configuration. 

The solubility of rare earth carbonates is fairly low and ranges from 10 to 10 mol L . Rare earth carbonates can be obtained by the addition of ammonium carbonate to a solution of a rare earth water-soluble salt. In this case, the precipitates will all be hydrates. Lanthanum to neodymium carbonates contain eight water molecules while neodymium to lutetium carbonates contain two water molecules only. Rare earth carbonates can be dissolved in alkali metal carbonate solutions and form a double salt of alkali metals. 

Figure 1.24 The structure of the complex [Nd2(C204)3-6H20]-4H20 [22]. (Reprinted from E. Hansson, Structural studies on the rare earth carboxylates 5. The crystal and molecnlar structure of neodymium (Ill)oxalate 10.5-hydrate, Acta Chemica Scandinavica, 24, 2969-2982, 1970, with permission from Forlagsforeningen Acta Chemica Scandinavica.)... 

The presence of water has been postulated to cause the thermal instability of many of the hydrates (3, 4, 20), and attempts to chromatograph neodymium (III) trifluoroacetylacetonate dihydrate have failed (23). There is some evidence that hydrolysis occurs at elevated temperatures (3, 4, 20). Furthermore, there are indications that certain complexes undergo hydrolysis when allowed to stand in vacuo, even at room temperature (20). For these reasons the chelates are difficult to dehydrate by conventional means. Many of the claims in the older literature that anhydrous tris chelates were obtained must be considered questionable because the assignments of composition were either arbitrary or were based on analytical methods relatively insensitive to the amount of water present. Some investigators, however, have reported reasonably well-characterized anhydrous tris complexes 4,11,15), but most of these are not sufficiently volatile and stable to be chromatographed. 

Neodymium has only one stable oxidation state (+3) within the electrochemical window of water, but it forms several relatively stable compounds with oxygen and hydrogen, which differ in their degree of hydration and in their crystallographic structure. Nominal degree of hydration indicated by a chemical name/formula reported in the literature does not necessarily reflect the actual degree of hydration. PZCs/IEPs of Neodymium (hydr)oxides are presented in Tables 3.689 through 3.691. 

Neodymium oxide was first isolated from a mixture of oxides called didymia. The elemeut ueodymium is the secoud most abuudaut lanthanide elemeut in the igneous rocks of Earth s crust. Hydrated neodymium(III) salts are reddish and anhydrous neodymium compounds are blue. The compounds neodymium(III) chloride, bromide, iodide, nitrate, perchlorate, and acetate are very soluble neodymium sulfate is somewhat soluble the fluoride, hydroxide, oxide, carbonate, oxalate, and phosphate compoimds are insoluble. 

In 1885 C. A. von Welsbach isolated two elements as oxides, praseodymium (the word meaning green twin ) and neodymium (meaning new twin ), from a mixture of lanthanide oxides called didymia. The oxides can be transformed to fluorides by reaction with HF at 700°C (1,292°F), or with NH4HF2 at 300°C (572°F). The hydrated fluorides are then dehydrated in vacuo in a current of HF gas. The metals praseodymium and neodymium are obtained via metallothermic reduction with calcium at approximately 1,450°C (2,642°F), or via electrolytic reduction of the melts. The metals can also be obtained from anhydrous chlorides, obtained via reaction of the oxides with ammonium chloride at 350°C (662 °F), which are then reduced with lithium-magnesium at approximately 100°C (212°F). 

Hydrated sulfato complexes of the type M M (S04)z JtH20 (Table 4) are known for M = U and Pu, and (NH4)2U2(S04)4-9H20 may be of the same type. KPu(S04)2-H20 and the corresponding dihydrate are isostructural with the neodymium(III) analogues and NH4Pu(S04)2 4H20 is isomorphous with the corresponding cerium(III) compound. Salts of other complex anions, such as KjM (S04)4 (M = Np, Pu), are also known. 

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