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	YCl3-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

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.:^[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

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.^[6]

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.^[7]

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.