Lanthanide metal trichlorides

Figure 7.10 Thermodynamic data (b)-(d) needed in analysis of the enthalpy of formation of the binary lanthanide metal compounds given in (a), (b) Sum of first, second and third ionization enthalpies of lanthanide metals (c) atomization enthalpy of lanthanide metals (d) derived lattice enthalpy of lanthanide trichlorides.

Before the availability of high-purity lanthanide metals, the most popular starting material was the oxide, readily available pure. Because of their high stability, the oxides cannot readily be converted into the respective trihalides simply by reaction with chlorine or hydrogen chloride as oxochlorides are formed nevertheless, Templeton and Carter (45) have prepared pure trichlorides using this method. 

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. 

Lanthanide triphenyl complexes have been known for about 40 years. These neutral triphenyl complexes [Ln(C6H5)3(THF) ] are synthesized by a transmetallation reaction of the lanthanide metal with diphenyl mercury or triphenyl bismuth in THF [34]. Recently, it has been reported that metathesis reaction of anhydrous lanthanide trichloride with aryllithium is a more convenient and reproducible method than the transmetallation reaction used previously for the synthesis of triaryl lanthanide complexes. Several lanthanide triphenyl and substituted triphenyl complexes have been synthesized (Equation 8.14) [35]. 

LANTHANIDE TRICHLORIDES BY REACTION OF LANTHANIDE METALS WITH MERCURY(II) CHLORIDE IN TETRAHYDROFURAN... 

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... 

Compounds of the type [(Me3Si)2N]3M have been prepared for all of the lanthanide elements except Pm, Tb, Dy, Tm, and Er (4). The synthetic method used in their preparation is nucleophilic substitution with three molar equivalents of lithium -or sodium - bis(trimethylsilyl)amide on the metal trichlorides in tetrahydrofuran. The compounds are rather high melting solids (145-170°C) which can be isolated by crystallization from pentane as long needles or by vacuum sublimation (80-100°C). The binary silylamides are monomeric in refluxing benzene solution, in the gas phase (by mass spectrometry), and in the solid state (by x-ray crystallography, see below). Thus, these compounds are three-coordinate, a unique coordination number for the lanthanide elements. 

Undoubtedly, the best method for the production of pure anhydrous lanthanide trihalides involves direct reaction of the elements. However, suitable reaction vessels, of molybdenum, tungsten, or tantalum, have to be employed silica containers result in oxohalides (27). Trichlorides have been produced by reacting metal with chlorine (28), methyl chloride (28), or hydrogen chloride (28-31). Of the tribromides, only that of scandium has been prepared by direct reaction with bromine (32). The triiodides have been prepared by reacting the metal with iodine (27, 29, 31, 33-41) or with ammonium iodide (42). 

The range of enthalpies of solution of anhydrous lanthanide trichlorides in water may be compared with those for other anhydrous chlorides. They are considerably more negative (Table IV) than those for the alkaline-earth metals [MgCl2, -155 kJ mol-1, to BaCl2, -13 kJ mol-1 (196)] and for the alkali metals [LiCl, — 37 kJ mol-1, to CsCl, +18 kJ mol-1 (197)]. 

Although several phenyl derivatives of the lanthanides and actinides have been characterized, only one re-arene complex of the / transition metals is known to date. This is the uranium(III) benzene complex, U(AlCl4)s CeHe 153), prepared by the combination of uranium tetrachloride, aluminum trichloride and aluminum powder in refluxing benzene, the Fischer-Hafner method [154). The molecular geometry of the complex is shown in Fig. 18. 

The lanthanide chloro precursors can be prepared in high yield by trans-metalation of the dilithiated ligands with anhydrous lanthanide trichlorides followed by work-up with diethyl ether. Alternatively, the mono-DME adducts... 

Tris(indenyl)lanthanide complexes of all rare earth metals can be prepared analogously from the lanthanide trichlorides with alkali or magnesium indenyl salts. All five-membered rings are -bonded to the metal. They form base adducts with... 

---