Rare trihalides

The hydrolysis of 106 was studied in some detail and led to the isolation and structural characterization of a rare example of a titanoxane derivative with a planar Ti2(p-G)2 core 108 [111]. The formation of complex 108 can be contrasted with the hydrolysis reaction of the trihalides in the Cp and Cp series giving rise... 

The preparations of rare-earth trihalides can be found in various books (2-8) and in Taylor s review (2 ). This review, however, did not include the preparation of scandium and yttrium trihalides, and only covered the preparation of the trifluorides very briefly. We have reviewed the preparation of all the trihalides (including scandium and yttrium) from Taylor s review up to June 1979 and have also included some methods and references missed by Taylor. Although we have mentioned all the methods available for the preparation of the trihalides, emphasis has been placed on the methods used since Taylor s review, and these have been referenced fully, whereas for the other methods, Taylor s review is recommended as a source of references. 

Rare-earth oxides dissolve in dilute hydrohalic acids to produce solutions of the trihalides which can be crystallized, giving six to nine waters of hydration depending upon the halide (cf. Table I). These hydrates cannot be thermally dehydrated, as oxohalides are formed ... 

Binary rare-earth compounds such as carbides, sulfides, nitrides, and hydrides have been used to prepare anhydrous trihalides, but they offer no special advantage. Treating these compounds at a high temperature with a halogen (98) or hydrogen halide (115) produces the trihalide, e.g.,... 

The hydrated trihalides of the rare earths are easily obtained by reacting the oxides with appropriate HX acid solution. Anhydrous halides are, however, difficult to prepare. Attempts to dehydrate the hydrated halides usually result in oxyhalides. In the case of the chlorides and bromides... 

Of the various crystal matrices, the cubic calcium fluoride (CaF2), the lanthanum trihalides (LaFs, LaCl3, LaBr3) and ethylsulphate are popular host lattices. The spectra of rare earths with partly filled /-shells in doped crystals consist of very sharp lines, similar to those in atomic spectra, of closely spaced groups. Fig. 20 gives a summary of the crystal... 

Various methods [282] have been used to prepare anhydrous chlorides of the rare earths. Taylor and Carter [283] describe a general method for the preparation of high purity anhydrous halides in good yield. This method involves heating in vacuo, a molecularly dispersed mixture of hydrated rare earth halide with proper ammonium halide until the water and ammonium halide are expelled. All the trihalides except the iodides of Sm and Eu can be obtained using this proceedure. In the case of Sm and Eu the divalent iodides, Sml2 and Eul2 are obtained. 

Lanthanide bromides and iodides have found important applications in a completely different field. They are added as additives in high-pressure discharge lamps in the lighting industry to improve the arc stability and the colour quality. The latter is due to the contribution of the multiline spectrum of the doped rare earths which are added to the salt mixture. Lanthanide trihalides of dysprosium, holmium, thullium, gadolinium and lutetium are used frequently for this purpose (Hilpert and Niemann, 1997). 

This volume of the Handbook illustrates the rich variety of topics covered by rare earth science. Three chapters are devoted to the description of solid state compounds skutteru-dites (Chapter 211), rare earth-antimony systems (Chapter 212), and rare earth-manganese perovskites (Chapter 214). Two other reviews deal with solid state properties one contribution includes information on existing thermodynamic data of lanthanide trihalides (Chapter 213) while the other one describes optical properties of rare earth compounds under pressure (Chapter 217). Finally, two chapters focus on solution chemistry. The state of the art in unraveling solution structure of lanthanide-containing coordination compounds by paramagnetic nuclear magnetic resonance is outlined in Chapter 215. The potential of time-resolved, laser-induced emission spectroscopy for the analysis of lanthanide and actinide solutions is presented and critically discussed in Chapter 216. 

Rare earth metal halides have received a great deal of attention during the past 50 years. The lanthanide trihalides are colored, magnetic salts, which have provided... 

Figure 19 Structures of the rare earth trihalides at ambient conditions...

Fluorides are preferred to chlorides because of the hygroscopic nature of the latter. Daane and Spedding showed that all the rare earth metals except Sm, Eu and Yb can be conveniently prepared by the reduction of their fluorides with calcium. In the case of Sm, Eu and Yb, the trihalides were reduced to dihalides only, no further reduction being acheived. 

A number of studies of rare-earth spectra in condensed phases (2, 4, 17, 18) have confirmed that the theory of Judd and of Ofelt can account for the observed intensities of electric-dipole transitions within the 4f configuration on the basis of three phenomenological parameters t2, t4, and tq. These studies have also shown that the parameter t2 is very sensitive to the rare-earth ion environment while t4 and tq are relatively insensitive to the environment. In the earlier gas-phase neodymium trihalide study, it was found that X2 is enhanced to such an extent relative to the t4 and tq parameters that the oscillator strength of the hypersensitive %/2 — C 5/2 transition is a factor of 10 higher than in condensed phases (JJ). 

In contrast to the plethora of Cr111 complexes, mononuclear Mo111 species are rare due to their susceptibility to oxidation and proclivity for dinucleation. However, like their Cr111 counterparts they are almost invariably octahedral in geometry and paramagnetic due to a high-spin d3 electron configuration. Halide complexes dominate this oxidation state and mer-trihalide species are heavily represented. Consequently, halide complexes are discussed ahead of other species.3... 

It should be noted here that not all these synthetic routes are equally well applicable to the rare earth elements. Route (b) is severely restricted by the paucity of simple lanthanide alkyls, while route (d) is unsuitable in the lanthanide case as lanthanide trihalides are generally unreactive towards Al,Al,Al -tris(trimethylsilyl)amidines. Deprotonation of amidines by metal amides should also be possible synthetic pathway since the delocalized structure of the resulting anion will increase the acidity. However, this route has apparently not yet been tried. 

Only a few X-ray diffraction and XAFS studies at high temperatures can be found in the literature mainly due to technical difficulties in experimental work. E.g. Okamoto et al. (1998, 1999) measured the X-ray diffraction of some molten rare earth and uranium trihalides. The obtained X-ray diffraction data were analyzed using molecular dynamics technique. This procedure is almost standard in the structural analysis of molten salt systems. 

Plausible though this mechanism is, it came under criticism (7) because, inter alia, it could not account for the intense hypersensitive transitions of the gaseous rare-earth trihalides (8). However, there is recent evidence that the halides are not planar (9,JL0), as had been previously supposed. If this is in fact the case, the importance of the mechanism based on Y terms in V remains undecided. 

The spectroscopic properties of several rare-earth trihalide-aluminum chloride complexes and various rare-earth chelates has been studied (9) and optical gain observed for a Nd-Al-Cl vapor complex (10). Measurements of the fluorescence kinetics show evidence of strong excited-state excited-state quenching. This plus the low molecular densities achievable reduce the attractiveness of these systems for practical laser applications. 

Compound formation in the alkali metal halide/rare earth metal trihalide systems seems to be limited to the four following formula types 1-4... 

Fig. 2. Compound formation in the alkali halide/rare-earth trihalide systems (chlorides and bromides) crystal structure known, see tables I and II (AjREXs tyPe compounds with A = K, Rb RE - La-Dy X = Cl, Br are isotypic with K2PTCI5, see G. Meyer and E. Hiittl, Z. Anorg. Allgem. (1983)) O observed in the phase diagram phase investigated by x-ray diffraction, crystal structure not known so far - not observed in the phase diagram no entry means that the respective phase diagram was not investigated.

The light yellow Lal3, melting point 778-779°, exhibits the PuBr3-type structure. The material is very sensitive to traces of both moisture and 02. The absence of cloudy appearance on the dissolution of Lal3 (and other rare earth metal trihalides) in absolute ethanol is not a good assurance of purity unless the material has been heated strongly to ensure the formation of crystalline LaOI (or other MOX phases) from absorbed moisture, hydroxide, etc. The same applies to the appearance of MOI in the powder pattern. 

The well-known Sm ry -arene complexes are generally prepared by the reaction of arenes with rare-earth trihalides in the presence of aluminium halides. This reaction was also effective with samarium diiodide and was extended to thulium diiodide however, an attempt to react Tml2 with naphthalene in toluene in the presence of aluminium trichloride was not successful but resulted in the isolation of an ry -toluene complex of Tm [77 -(CH3C6H5)Tm(AlCl4)3] (Figure 6) (Fagin et al., 2005). 

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