# Lanthanide trichloride

Lanthanide trichlorides are a family of inorganic compound with the formula LnCl$_3$, 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 :
| MCl$_3$ | color | structure type | f-configuration | comment |
| ScCl$_3$ | colorless | AlCl$_3$-type | f$^0$ | not classified as a lanthanide usually |
| YCl$_3$ | colorless | AlCl$_3$-type | f$^0$ | not classified as a lanthanide usually |
| LaCl$_3$ | colorless | UCl$_3$-type | f$^0$ | diamagnetic |
| CeCl$_3$ | colorless | UCl$_3$-type | f$^1$, doublet | - |
| PrCl$_3$ | green | UCl$_3$-type | f$^2$, triplet | - |
| NdCl$_3$ | pink | UCl$_3$-type | f$^3$, quartet | - |
| PmCl$_3$ | green | UCl$_3$-type | f$^4$, quintet | radioactive |
| SmCl$_3$ | yellow | UCl$_3$-type | f$^5$, sextet | - |
| EuCl$_3$ | yellow | UCl$_3$-type | f$^6$, septet | - |
| GdCl$_3$ | colorless | UCl$_3$-type | f$^7$, octet | symmetrical electronic shell |
| TbCl$_3$ | white | PuBr$_3$-type | f$^8$, septet | - |
| DyCl$_3$ | white | AlCl$_3$-type | f$^9$, sextet | - |
| HoCl$_3$ | yellow | AlCl$_3$-type | f$^{10}$, quintet | - |
| ErCl$_3$ | violet | AlCl$_3$-type | f$^{11}$, quartet | - |
| TmCl$_3$ | yellow | AlCl$_3$-type | f$^{12}$, triplet | - |
| YbCl$_3$ | colorless | YCl$_3$-type | f$^{13}$, doublet | - |
| LuCl$_3$ | colorless | AlCl$_3$-type | f$^{14}$ | diamagnetic |

## Preparation

The lanthanide oxides and carbonates dissolve in hydrochloric acid to give chloride salt of the hydrated cations:
    M$_2$O$_3$ + 6 HCl + n H$_2$O → 2 [Ln(H$_2$O)$_n$]Cl$_3$

### Industrial routes

Anhydrous trichlorides are produced commercially by carbothermic reaction of the oxide:
    M$_2$O$_3$ + 3 Cl$_2$ + 3 C → 2 MCl$_3$ + 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:
    M$_2$O$_3$ + 10 NH$_4$Cl → 2 (NH$_4$)$_2$MCl$_5$ + 3 H$_2$O + 6 NH$_3$
(M = Dy, Ho, Er, Tm, Lu, Yb, Y, Sc)
    Tb$_4$O$_7$ + 22 NH$_4$Cl → 4 (NH$_4$)$_2$TbCl$_5$ + 7 H$_2$O + 14 NH$_3$
Other lanthanides for hexachlorides:
    M$_2$O$_3$ + 12 NH$_4$Cl → 2 (NH$_4$)$_3$MCl$_6$ + 3 H$_2$O + 6 NH$_3$
(M = La, Ce, Nd, Pm, Sm, Eu, Gd)
    Pr$_6$O$_{11}$ + 40 NH$_4$Cl → 6 (NH$_4$)$_3$PrCl$_6$ + 11 H$_2$O + 22 NH$_3$
These reactions can also start with the metals, e.g.:
    Y + 5 NH$_4$Cl → (NH$_4$)$_2$YCl$_5$ + 1.5 H$_2$ + 3 NH$_3$

#### 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:
    (NH$_4$)$_2$MCl$_5$ → MCl$_3$ + 2 HCl + 2 NH$_3$
    (NH$_4$)$_3$MCl$_6$ → MCl$_3$ + 3 HCl + 3 NH$_3$

### 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, UCl$_3$ and YCl$_3$. The UCl$_3$ structure features 9-coordinate metal centers. The PuBr3 structure, adopted uniquely by TbCl$_3$, 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:
    LnCl$_3$ + Al → Ln + AlCl$_3$
In some cases, the trifluoride is preferred.
They react with humid air to give oxychlorides:
    LnCl$_3$ + H$_2$O → LnOCl + 2 HCl
For synthetic chemists, this reaction is a problematic since the oxychlorides are less reactive.