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Lanthanide trichlorides are a family of inorganic compound with the formula LnCl₃, where Ln stands for a lanthanide metal. The trichlorides are standard reagents in applied and academic chemistry of the lanthanides. They exist as anhydrous solids and as hydrates.

Properties

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.

Preparation

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

M₂O₃ + 6 HCl + n H₂O → 2 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)

Pr₆O₁₁ + 40 NH₄Cl → 6 (NH₄)₃PrCl₆ + 11 H₂O + 22 NH₃

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

Other methods

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

Structures

As indicated in the table, the 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.

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.

References

1. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
2. ^ I. McGill (2005). "Rare Earth Elements". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a22_607. ISBN 978-3527306732.
3. ^ Brauer, G., ed. (1963). Handbook of Preparative Inorganic Chemistry (2nd ed.). New York: Academic Press.
4. ^ Meyer, G. (1989). "The Ammonium Chloride Route to Anhydrous Rare Earth Chlorides—The Example of Ycl ₃". The Ammonium Chloride Route to Anhydrous Rare Earth Chlorides-The Example of YCl₃. Inorganic Syntheses. Vol. 25. pp. 146–150. doi:10.1002/9780470132562.ch35. ISBN 978-0-470-13256-2.
5. Edelmann, F. T.; Poremba, P. (1997). Herrmann, W. A. (ed.). Synthetic Methods of Organometallic and Inorganic Chemistry. Vol. VI. Stuttgart: Georg Thieme Verlag. ISBN 978-3-13-103021-4.
6. Habenschuss, A.; Spedding, F. H. (1980). "Dichlorohexaaquagadolinium(III) Chloride (GdCl₂(H₂O)₆)C". Crystal Structure Communications. 9: 213-218.{{cite journal}}: CS1 maint: multiple names: authors list (link)
7. Cotton, Simon A. (2011). "Scandium, Yttrium & the Lanthanides: Inorganic & Coordination Chemistry". Encyclopedia of Inorganic and Bioinorganic Chemistry. doi:10.1002/9781119951438.eibc0195. ISBN 9781119951438.

• Chlorides
• Lanthanum compounds
• Lanthanide halides