Lanthanide chlorides

Lanthanide chlorides are inorganic compounds that contain a lanthanide (Ln) and chloride. The simplest members, the Lanthanide trichlorides, have the formula LnCl3. Important derivatives are the hydrates with the formula LnCl3(H2O)n. Various ether complexes are also well known. Myriad mixed ligand complexes are known.

Lanthanide(III) chlorides The anhydrous solids have melting points range from ca. 582 (Tb) - 925 °C (Lu). They are generally pale colored, often white. As coordination polymers, they only dissolve in donor solvents, including water.

In terms of their structures, anhydrous trichlorides follow two main motifs, UCl3 and YCl3. The UCl3 structure features 9-coordinate metal centers. The PuBr3 structure, adopted uniquely by TbCl3, features 8-coordinated metals. The remaining later metals are 6-coordinate as is aluminium trichloride. In the gas phase the trihalides are planar or approximately planar, the lighter lanthanides have a lower % of dimers, the heavier lanthanides a higher proportion. The dimers have a similar structure to Al2Cl6.

Reactions Lanthanide trichlorides are commercial precursors to the metals by reduction, e.g. with aluminium:

LnCl3 + Al → Ln + AlCl3 In some cases, the trifluoride is preferred. They react with humid air to give oxychlorides:

LnCl3 + H2O → LnOCl + 2 HCl For synthetic chemists, this reaction is a problematic since the oxychlorides are less reactive.

Preparation The lanthanide oxides and carbonates dissolve in hydrochloric acid to give chloride salt of the hydrated cations:

M2O3 + 6 HCl + n H2O → 2 [Ln(H2O)n]Cl3

Industrial routes Anhydrous trichlorides are produced commercially by carbothermic reaction of the oxide:

M2O3 + 3 Cl2 + 3 C → 2 MCl3 + 3 CO

Ammonium chloride route The ammonium chloride route refers to a general procedure to produce anhydrous lanthanide chlorides. The method has the advantages of being general for the 14 lanthanides and it produces air-stable intermediates that resist hydrolysis. The use of ammonium chloride as a reagent is convenient because the salt is anhydrous, even when handled in air. Ammonium chloride is also attractive because it thermally decomposes to volatile products at temperatures compatible with the stability of the trichloride targets.

Step 1 preparation of ammonium lanthanide chlorides The reaction of an intimate mixture of lanthanide oxides with excess ammonium chloride produces anhydrous ammonium salts of the penta- and hexachlorides. Typical reaction conditions are hours at 230-250 °C. Some lanthanides (as well as scandium and yttrium) form pentachlorides:

M2O3 + 10 NH4Cl → 2 (NH4)2MCl5 + 3 H2O + 6 NH3 (M = Dy, Ho, Er, Tm, Lu, Yb, Y, Sc)

Tb4O7 + 22 NH4Cl → 4 (NH4)2TbCl5 + 7 H2O + 14 NH3 Other lanthanides for hexachlorides:

M2O3 + 12 NH4Cl → 2 (NH4)3MCl6 + 3 H2O + 6 NH3 (M = La, Ce, Nd, Pm, Sm, Eu, Gd) These reactions can also start with the metals, e.g.:

Y + 5 NH4Cl → (NH4)2YCl5 + 1.5 H2 + 3 NH3 Step 2 thermolysis of ammonium lanthanide chlorides The ammonium lanthanum chlorides are converted to the trichlorides by heating in a vacuum. Typical reaction temperatures are 350–400 °C:

(NH4)2MCl5 → MCl3 + 2 HCl + 2 NH3 (NH4)3MCl6 → MCl3 + 3 HCl + 3 NH3 One-pot route In the preparation of PrCl3 from its oxide, intermediates need not be isolated. The proposed stoichiometry follows:

Pr6O11 + 22 NH4Cl → 6 PrCl3 + 22 NH3 + 11 H2O + 2 Cl2

Other methods Hydrated lanthanide trichlorides dehydrate under a hot stream of hydrogen chloride.

Lanthanide dichlorides Examples include neodymium dichloride, samarium dichloride, europium(II) chloride, dysprosium dichloride, thulium dichloride, and ytterbium dichloride. They can be prepared by reducing the trivalent chloride with lithium metal/naphthalene in tetrahydrofuran:

LnCl3 + Li → LnCl2 + LiCl Reducing the chloride with the metal or hydrogen is also possible:

2 LnCl3 + Ln → 3 LnCl2 (Ln=Nd,Sm,Eu?,Dy,Tm,Yb) 2 LnCl3 + H2 → 2 LnCl2 + 2 HCl (Ln=Nd,Sm,Eu,Dy,Tm,Yb)

See also Lanthanide Chloride Lanthanide tribromide

References