Lanthanide trichlorides are a family of

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...

with the

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwee ...

Ln Cl₃, 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 + 6HCl + n H₂O → 2 n(H₂O)_nl₃

Industrial routes

Anhydrous trichlorides are produced commercially by

carbothermic reaction Carbothermic reactions involve the reduction of substances, often metal oxides (O^2-), using carbon as the reducing agent. These chemical reactions are usually conducted at temperatures of several hundred degrees Celsius. Such processes are applie ...

of the oxide: :M₂O₃ + 3Cl₂ + 3C → 2MCl₃ + 3CO

Ammonium chloride route

The ammonium chloride route refers to a general procedure to produce

anhydrous A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achie ...

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 Scandium is a chemical element with the symbol Sc and atomic number 21. It is a silvery-white metallic d-block element. Historically, it has been classified as a rare-earth element, together with yttrium and the Lanthanides. It was discovered in ...

and

yttrium Yttrium is a chemical element with the symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and has often been classified as a "rare-earth element". Yttrium is almost always found in com ...

) form ''penta''chlorides: :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 ''hexa''chlorides: :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.: :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

Other methods

Hydrated lanthanide trichlorides dehydrate under a hot stream of

hydrogen chloride The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chloride ga ...

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 Aluminium chloride, also known as aluminium trichloride, is an inorganic compound with the formula . It forms hexahydrate with the formula , containing six water molecules of hydration. Both are colourless crystals, but samples are often contam ...

Reactions

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

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...

: :LnCl₃ + Al → Ln + AlCl₃ In some cases, the trifluoride is preferred. They react with humid air to give

oxychloride In chemistry, molecular oxohalides (oxyhalides) are a group of chemical compounds in which both oxygen and halogen atoms are attached to another chemical element A in a single molecule. They have the general formula , where X = fluorine (F), chlor ...

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

References

{{Chlorides Chlorides Lanthanum compounds Lanthanide halides