Hydrated trihalides

One of the oldest methods is to pass HX gas over the heated hydrated trihalide. Thus, hydrated trichlorides (61, 88-92) are heated initially to 105°C, and then when most of the water has been removed the temperature is raised to 350°C. This method works well for trichlorides, but it does not produce pure tribromides (93-95) or triiodides better results are obtained when the triiodides are heated in a flow of HI and... 

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... 

Rowley, A. T. et al., Inorg. Chem. Acta, 1993, 211(1), 77 Preparation of metal oxides by fusing metal halides with lithium oxide in a sealed tube leads to explosions if halide hydrates are employed, particularly lanthanide trihalide hydrates. The preparation succeeds with anhydrous halides. This will be purely a question of vapour pressure above an exothermic reaction the question is whether the vapour is water, or metal halide, and the reaction oxide formation, or hydration of lithium oxide. Like other alkali metal oxides, hydration is extremely energetic. 

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 ... 

Water is, of course, a particularly difficult solvent to remove from solvates (i.e., hydrates). Thermal decomposition patterns for solvates containing such solvents as tetrahydrofuran suggest that thermal treatment of such solvates may prove a feasible route to anhydrous trihalides. 

Hproblems associated with all the trihalides of this review of the presence of small amounts of hydrates or oxochlorides. While on the matter of possible impurities, it may be recalled that in Bommer and Hohmann s early work there is a discrepancy between enthalpies of solution of anhydrous trichlorides and of respective metals in hydrochloric acid. Here the more likely impurity to be responsible is unreacted potassium metal in the lanthanide metal used in the hydrochloric acid dissolution experiments. 

In principle, Gibbs free energies of transfer for trihalides can be obtained from solubilities in water and in nonaqueous or mixed aqueous solutions. However, there are two major obstacles here. The first is the prevalence of hydrates and solvates. This may complicate the calculation of AGtr(LnX3) values, for application of the standard formula connecting AGt, with solubilities requires that the composition of the solid phase be the same in equilibrium with the two solvent media in question. The other major hurdle is that solubilities of the trichlorides, tribromides, and triiodides in water are so high that knowledge of activity coefficients, which indeed are known to be far from unity 4b), is essential (201). These can, indeed, be measured, but such measurements require much time, care, and patience. 

Trivalent Am " ions occur in aqueous acid solution. The solution has a pink color and the ion exists as a hydrated species. Reactions with halide salts or the acids produce trihalides. 

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. 

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. 

Preparation of metal oxides by fusing metal halides with lithium oxide in a sealed tube leads to explosions if halide hydrates are employed, particularly lanthanide trihalide... 

The general chemistry of Ac3 in both solid compounds and solution, where known, is very similar to that of lanthanum, as would be expected from the similarity in position in the Periodic Table and in radii (Ac3, 1.10 La3, 1.06 A) together with the noble gas structure of the ion. Thus actinium is a true member of Group 3, the only difference from lanthanum being in the expected increased basicity. The increased basic character is shown by the stronger absorption of the hydrated ion on cation-exchange resins, the poorer extraction of the ion from concentrated nitric acid solutions by tributyl phosphate, and the hydrolysis of the trihalides with water vapor at 1000°C to the oxohalides AcOX the lanthanum halides are hydrolyzed to oxide by water vapor at 1000°C. 

In the case of AnCls, the route used for lanthanide trihalides involving heating the oxide or hydrated chloride with ammonium chloride works well ... 

Generally, the reduction of aryl tellurium trihalides by a variety of reducing agents leads to diaryl ditellurium compounds. However, the reduction of aryl tellurium trihalides with stabilizing groups in the or//to-position to the tellurium atom, by sodium hydrogen sulfite, disodium disulfite, or hydrazine hydrate " to aryl tellurium halides shows that the reduction can be stopped at the tellurium halide stage. 

Aryl tellurium trihalides are easily reduced by sodium hydrogen sulfite , sodium or potassium disulfitezinc in refluxing ethanoP, hydrazine hydrate " hypophos-phorous acid °, sodium sulfide nonahydrate , sodium borohydride , thiourea diox-ide, 4-methylbenzenesulfonyl hydrazide , or diphenylsilane. The reductions with sulfites are generally carried out in an aqueous medium in the presence of an immiscible organic solvent that extracts the ditellurium. 

Anhydrous lanthanide trihalides, particularly the trichlorides, are important reactants for the formation of a variety of lanthanide complexes, including organometallics. Routes for the syntheses of anhydrous lanthanide trihalides generally involve high temperature procedures or dehydration of the hydrated halides.The former are inconvenient and complex for small scale laboratory syntheses, while dehydration methods may also be complex and have limitations, for example, use of thionyl chloride. - Moreover, the products from these routes may require purification by vacuum sublimation at elevated temperatures. Redox transmetalation between lanthanide metals and mercury(II) halides was initially carried out at high temperatures. However, this reaction can be carried out in tetrahydrofuran (THF, solvent) to give complexes of lanthanide trihalides with the solvent. These products are equally as suitable as reactants for synthetic purposes as the uncomplexed... 

An electrostatic hydration model, previously developed for ions of the noble gas structure, has been applied to the tervalent lanthanide and actinide ions. For lanthanides the application of a single primary hydration number resulted in a satisfactory fit of the model to the experimentally determined free energy and enthalpy data. The atomization enthalpies of lanthanide trihalide molecules have been calculated in terms of a covalent model of a polarized ion. Comparison with values obtained from a hard sphere modeP showed that a satisfactory description of the bonding in these molecules must ultimately be formulated from the covalent perspective. 

---