Lanthanide trichloride

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^[1]
MCl₃	color	structure type	f-configuration	comment
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	diamagnetic
CeCl₃	colorless	UCl₃-type	f¹, doublet	-
PrCl₃	green	UCl₃-type	f², triplet	-
NdCl₃	pink	UCl₃-type	f³, quartet	-
PmCl₃	green	UCl₃-type	f⁴, quintet	radioactive
SmCl₃	yellow	UCl₃-type	f⁵, sextet	-
EuCl₃	yellow	UCl₃-type	f⁶, septet	-
GdCl₃	colorless	UCl₃-type	f⁷, octet	symmetrical electronic shell
TbCl₃	white	PuBr₃-type	f⁸, septet	-
DyCl₃	white	AlCl₃-type	f⁹, sextet	-
HoCl₃	yellow	AlCl₃-type	f¹⁰, quintet	-
ErCl₃	violet	AlCl₃-type	f¹¹, quartet	-
TmCl₃	yellow	AlCl₃-type	f¹², triplet	-
YbCl₃	colorless	AlCl₃-type	f¹³, doublet	-
LuCl₃	colorless	AlCl₃-type	f¹⁴	diamagnetic

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:^[2]

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.^[3]^[4]^[5]

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.^[4] 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^{[dubious – discuss]}

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.:^[4]

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:^[4]

(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:^[6]

Pr₆O₁₁ + 22 NH
4Cl → 6 PrCl
3 + 22 NH
3 + 11 H
2O + 2 Cl
2

Other methods

Hydrated lanthanide trichlorides dehydrate under a hot stream of hydrogen chloride.^[3]

Structures

Structure of GdCl₃^.6H₂O, which consists of [GdCl₂(H₂O)₆]⁺ centers. The coordination spheres are interconnected by hydrogen bonds between the protons and both the coordinated and the ionic chlorides.^[7]

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.^[8]

Reactions

Lanthanide trichlorides are commercial precursors to the metals by reduction, e.g. with aluminium:^[2]

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

↑ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8. Search this book on
↑ ^2.0 ^2.1 I. McGill (2005). "Rare Earth Elements". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a22_607.
↑ ^3.0 ^3.1 K. Wetzel (1963). "Rare Earth Trichlorides". In G. Brauer. Handbook of Preparative Inorganic Chemistry, 2nd Ed. 2. NY, NY: Academic Press. p. 1146. Search this book on
↑ ^4.0 ^4.1 ^4.2 ^4.3 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. 25. pp. 146–150. doi:10.1002/9780470132562.ch35. ISBN 978-0-470-13256-2. Search this book on
↑ Edelmann, F. T.; Poremba, P. (1997). Herrmann, W. A., ed. Synthetic Methods of Organometallic and Inorganic Chemistry. VI. Stuttgart: Georg Thieme Verlag. ISBN 978-3-13-103021-4. Search this book on
↑ Mendil, Fatsiha; Berkani, Madjid; Zamouche, Abdelmalek; Rycerz, Leszek (2013). "Determination of the optimum conditions for the synthesis of praseodymium(III) chloride". Comptes Rendus. Chimie. 16 (9): 795–798. doi:10.1016/j.crci.2013.02.017.
↑ Habenschuss, A.; Spedding, F. H. (1980). "Dichlorohexaaquagadolinium(III) Chloride (GdCl₂(H₂O)₆)C". Crystal Structure Communications. 9: 213–218.CS1 maint: Multiple names: authors list (link)
↑ Cotton, Simon A. (2011). "Scandium, Yttrium & the Lanthanides: Inorganic & Coordination Chemistry". Encyclopedia of Inorganic and Bioinorganic Chemistry. doi:10.1002/9781119951438.eibc0195. ISBN 9781119951438. Search this book on

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[1] Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8. Search this book on

[Ull-2] 2.0 ^2.1 I. McGill (2005). "Rare Earth Elements". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a22_607.

[Brauer-3] 3.0 ^3.1 K. Wetzel (1963). "Rare Earth Trichlorides". In G. Brauer. Handbook of Preparative Inorganic Chemistry, 2nd Ed. 2. NY, NY: Academic Press. p. 1146. Search this book on

[IS-4] 4.0 ^4.1 ^4.2 ^4.3 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. 25. pp. 146–150. doi:10.1002/9780470132562.ch35. ISBN 978-0-470-13256-2. Search this book on

[EdelmannPoremba1997-5] Edelmann, F. T.; Poremba, P. (1997). Herrmann, W. A., ed. Synthetic Methods of Organometallic and Inorganic Chemistry. VI. Stuttgart: Georg Thieme Verlag. ISBN 978-3-13-103021-4. Search this book on

[6] Mendil, Fatsiha; Berkani, Madjid; Zamouche, Abdelmalek; Rycerz, Leszek (2013). "Determination of the optimum conditions for the synthesis of praseodymium(III) chloride". Comptes Rendus. Chimie. 16 (9): 795–798. doi:10.1016/j.crci.2013.02.017.

[7] Habenschuss, A.; Spedding, F. H. (1980). "Dichlorohexaaquagadolinium(III) Chloride (GdCl₂(H₂O)₆)C". Crystal Structure Communications. 9: 213–218.CS1 maint: Multiple names: authors list (link)

[8] Cotton, Simon A. (2011). "Scandium, Yttrium & the Lanthanides: Inorganic & Coordination Chemistry". Encyclopedia of Inorganic and Bioinorganic Chemistry. doi:10.1002/9781119951438.eibc0195. ISBN 9781119951438. Search this book on

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