Trihalides anhydrous salts

More care has to be taken for the preparation of binary chlorides and bromides, RCI3 and RBr3. The anhydrous salts are moisture-sensitive and they are obtained as (hepta- or hexa-) hydrates upon crystallization from hydrochloric or hydrobromic acid solutions. Simple dehydration of the hydrates in an HCl or HBr gas flow, respectively, appears to be possible (Seifert et al. 1985) but the less skfllliil chemist produces halide products more or less contaminated with oxyhahde, ROX. Sublimation of the raw product in an all-tantalum apparatus at higher temperatures (650 C to 950 C) and low pressures (10 bar) produces pure trihalides (with the exception of the rare-earth elements europium and 3dterbium where mixed-valent (+2/+3) or even dihalides are obtained under these conditions). 

The oxides have been converted to the trihalides by reaction with amine hydrohalides with mp <200°C (e.g., PhNH2, MeNH2, Me2NH, EtNH2, Et NH, etc.). A double salt was formed from the reaction with the hydrohalide, which served both as solvent and halogenating agent. The reaction mixture was heated to vaporize the solvent and decompose the double salt, leaving the anhydrous halide (85). 

Comparisons of solubilities of trichlorides, tribromides, and triiodides of the lanthanides in a variety of nonaqueous solvents can be found in a Russian review (310). Perhaps the widest range of solubilities of lanthanide(III) salts in nonaqueous media refers not to the trihalides but to the nitrates, whose solubilities in 31 solvents have been measured (312). Unfortunately, these measurements were carried out on the hexahydrates rather than anhydrous materials. 

The conventional preparative routes to anionic, neutral, or cationic complexes of indium start with the metal, which is dissolved in a suitable mineral acid to give a solution from which hydrated salts can be obtained by evaporation. These hydrates react with a variety of neutral or anionic ligands in nonaqueous solvents, and a wide range of indium(III) complexes have been prepared in this manner.1 Alternatively, the direct high-temperature oxidation of the metal by halogens yields the anhydrous trihalides, which are again convenient starting materials in synthetic work. In the former case, the initial oxidation of the metal is followed by isolation, solution reaction, precipitation, and recrystallization. 

Not all metals are capable of forming polyhalide salts. Although considerable research remains to be done in this field, it seems at the present time that in order to form a polyhalide salt the cation must have a large radius and a small charge. The best known and the most stable of the metal polyhalogen salts are those of cesium and rubidium. The potassium salts are more difficult to form and are somewhat more unstable in the solid state. The sodium and lithium polyhalides have not been prepared in anhydrous form. Polyvalent cations apparently do not yield salts with the trihalides, f However, salts of magnesium, zinc,... 

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