Lanthanide trichloride explained

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.

Lanthanide trichlorides

MCl3	color	structure type	f-configuration	comment
	colorless	AlCl3-type	f^0	not classified as a lanthanide usually
	colorless	AlCl3-type	f^0	not classified as a lanthanide usually
	colorless	UCl3-type	f^0	diamagnetic
	colorless	UCl3-type	f^1, doublet	-
	green	UCl3-type	f^2, triplet	-
	pink	UCl3-type	f^3, quartet	-
	green	UCl3-type	f^4, quintet	radioactive
	yellow	UCl3-type	f^5, sextet	-
	yellow	UCl3-type	f^6, septet	-
	colorless	UCl3-type	f^7, octet	symmetrical electronic shell
	white	PuBr3-type	f^8, septet	-
	white	AlCl3-type	f^9, sextet	-
	yellow	AlCl3-type	f^10, quintet	-
	violet	AlCl3-type	f^11, quartet	-
	yellow	AlCl3-type	f^12, triplet	-
	colorless	YCl3-type	f^13, doublet	-
	colorless	AlCl3-type	f^14	diamagnetic

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.^{[1] [2] [3]}

Step 1: preparation of ammonium lanthanide chloridesThe 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.^[2] 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)

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

These reactions can also start with the metals, e.g.:^[2]

Y + 5NH₄Cl → (NH₄)₂YCl₅ + 1.5H₂ + 3NH₃

Step 2: thermolysis of ammonium lanthanide chloridesThe ammonium lanthanum chlorides are converted to the trichlorides by heating in a vacuum. Typical reaction temperatures are 350–400 °C:^[2]

(NH₄)₂MCl₅ → MCl₃ + 2HCl + 2NH₃

(NH₄)₃MCl₆ → MCl₃ + 3HCl + 3NH₃

Other methods

Hydrated lanthanide trichlorides dehydrate under a hot stream of hydrogen chloride.^[1]

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.^[4]

Reactions

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

LnCl₃ + Al → Ln + AlCl₃In some cases, the trifluoride is preferred.

They react with humid air to give oxychlorides:

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

Notes and References

1. Book: Handbook of Preparative Inorganic Chemistry. 2nd. Brauer, G. . Academic Press. 1963. New York.
2. Book: Meyer , G. . The Ammonium Chloride Route to Anhydrous Rare Earth Chlorides-The Example of YCl₃. The Ammonium Chloride Route to Anhydrous Rare Earth Chlorides—The Example of Ycl ₃. Inorganic Syntheses. 25. 1989. 146–150. 10.1002/9780470132562.ch35. 978-0-470-13256-2.
3. Book: Edelmann, F. T. . Synthetic Methods of Organometallic and Inorganic Chemistry . VI . Poremba, P. . Herrmann, W. A. . 1997 . Georg Thieme Verlag . Stuttgart . 978-3-13-103021-4 .
4. Book: 10.1002/9781119951438.eibc0195. Scandium, Yttrium & the Lanthanides: Inorganic & Coordination Chemistry. Encyclopedia of Inorganic and Bioinorganic Chemistry. 2011. Cotton. Simon A.. 9781119951438.

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