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) and related 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. + Lanthanide(III) trichlorides and related compounds MCl3 color structure type f-configurationhydrateTHF complexescomments ScCl3 colorless AlCl3-type f0 not classified as a lanthanide usually YCl3 colorless AlCl3-type f0 not classified as a lanthanide usually LaCl3 colorless UCl3-type f0[LaCl(H2O)7]2Cl4[LaCl3(thf)2] = [La(μ-Cl)3(thf)2]n diamagnetic CeCl3 colorless UCl3-type f1, doublet [CeCl(H2O)7]2Cl4[CeCl3(thf)2] = [Ce(μ-Cl)2Cl(thf)2]n; [CeCl3(thf)4]- PrCl3 green UCl3-type f2, triplet[PrCl2(H2O)6]Cl[PrCl3(thf)2] = [Pr(μ-Cl)2Cl(thf)2]n- NdCl3 pink UCl3-type f3, quartet [NdCl2(H2O)6]Cl [NdCl3(thf)4], [NdCl(μ-Cl)2(thf)2]n, PmCl3 green UCl3-type f4, quintet [PmCl3(thf)2]n radioactive SmCl3 yellow UCl3-type f5, sextet [SmCl2(H2O)6]Cl [SmCl3(thf)4] EuCl3 yellow UCl3-type f6, septet [EuCl2(H2O)6]Cl EuCl3(thf)4 GdCl3 colorless UCl3-type f7, octet [GdCl2(H2O)6]Cl [GdCl3(thf)3.5] = [GdCl2(thf)5]+[GdCl4(thf)2]-, [GdCl3(thf)4, [GdCl3(thf)2]nsymmetrical electronic shell TbCl3 white PuBr3-type f8, septet [TbCl2(H2O)6]Cl [TbCl3(thf)3·5] = [TbCl2(thf)5]+[TbCl4(thf)2]- DyCl3 white AlCl3-type f9, sextet [DyCl2(H2O)6]Cl [DyCl3(thf)3·5] = [DyCl2(thf)5]+[DyCl4(thf)2]- HoCl3 yellow AlCl3-type f10, quintet[HoCl2(H2O)6]Cl [HoCl3(thf)3·5] = [HoCl2(thf)5]+[HoCl4(thf)2]- ErCl3 violet AlCl3-type f11, quartet [ErCl2(H2O)6]Cl [ErCl3(thf)3·5] = [ErCl2(thf)5]+[ErCl4(thf)2]- TmCl3 yellow AlCl3-type f12, triplet[TmCl2(H2O)6]Cl YbCl3 colorless YCl3-type f13, doublet [YbCl2(H2O)6]Cl [YbCl3(thf)3·5] = [YbCl2(thf)5]+[YbCl4(thf)2]-, YbCl3(THF)3, Yb2(μ-Cl)2Cl4(THF)4 LuCl3 colorless AlCl3-type f14[LuCl2(H2O)6]Cl LuCl3(thf)3diamagnetic

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 + 6HCl + n H2O → 2[Ln(H2O)n]Cl3

Industrial routes Anhydrous trichlorides are produced commercially by carbothermic reaction of the oxide: M2O3 + 3Cl2 + 3C → 2MCl3 + 3CO

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 + 10NH4Cl → 2(NH4)2MCl5 + 3H2O + 6NH3 (M = Dy, Ho, Er, Tm, Lu, Yb, Y, Sc) Tb4O7 + 22NH4Cl → 4(NH4)2TbCl5 + 7H2O + 14NH3

Other lanthanides for hexachlorides: M2O3 + 12NH4Cl → 2(NH4)3MCl6 + 3H2O + 6NH3 (M = La, Ce, Nd, Pm, Sm, Eu, Gd)

These reactions can also start with the metals, e.g.: Y + 5NH4Cl → (NH4)2YCl5 + 1.5H2 + 3NH3

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 + 2HCl + 2NH3 (NH4)3MCl6 → MCl3 + 3HCl + 3NH3

One-pot route In the preparation of PrCl3 from its oxide, intermediates need not be isolated. The proposed stoichiometry follows:

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