Praseodymium(III) chloride

Praseodymium chloride is the inorganic compound with the formula Pr Cl₃. Like other lanthanide trichlorides, it exists both in the anhydrous and hydrated forms. It is a blue-green solid that rapidly absorbs water on exposure to moist air to form a light green heptahydrate.

Preparation

Praseodymium chloride is prepared by treating praseodymium metal with hydrogen chloride:
It is usually purified by vacuum sublimation.
Hydrated salts of praseodymium chloride can be prepared by treatment of either praseodymium metal or praseodymium carbonate with hydrochloric acid:
PrCl₃∙7H₂O is a hygroscopic substance, that will not crystallize from the mother liquor unless it is left to dry in a desiccator. Anhydrous PrCl₃ can be made by thermal dehydration of the hydrate at 400 °C in the presence of ammonium chloride, the so-called ammonium chloride route. Alternatively the hydrate can be dehydrated using thionyl chloride.

Praseodymium chloride is Lewis acidic, classified as "hard" according to the HSAB concept. Rapid heating of the hydrate may cause small amounts of hydrolysis. PrCl₃ forms a stable Lewis acid-base complex K₂PrCl₅ by reaction with potassium chloride; this compound shows interesting optical and magnetic properties.
Aqueous solutions of praseodymium chloride can be used to prepare insoluble praseodymium compounds. For example, praseodymium phosphate and praseodymium fluoride can be prepared by reaction with potassium phosphate and sodium fluoride, respectively:
When heated with alkali metal chlorides, it forms a series of ternary materials with the formulae MPr₂Cl₇, M₃PrCl₆, M₂PrCl₅, and M₃Pr₂Cl₉ where M = K, Rb, Cs.