Praseodymium(III) chloride

Praseodymium(III) chloride (PrCl3), also known as praseodymium trichloride, is a compound of praseodymium and chlorine. It is a blue-green solid which rapidly absorbs water on exposure to moist air to form a light green heptahydrate.

Chemical properties
 Praseodymium(III) chloride is a moderately strong Lewis acid, which ranks 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(III) chloride can be used to prepare insoluble praseodymium(III) compounds, for example praseodymium(III) phosphate or praseodymium(III) fluoride:

PrCl3(aq) +  K3PO4(aq)  →  PrPO4(s)  +  3 KCl(aq)

PrCl3(aq) +  3 NaF(aq) →  PrF3(s)  +  3 NaCl(aq)

Preparation
 Praseodymium(III) chloride can be prepared as a green aqueous solution by reaction of either praseodymium metal or praseodymium(III) carbonate and hydrochloric acid. The anhydrous halide may alternatively be prepared from praseodymium metal and hydrogen chloride.

2 Pr(s) +  6 HCl(aq)  →  2 PrCl3(aq)  +  3 H2(g)

Pr2(CO3)3(s) +  6 HCl(aq)  →  2 PrCl3(aq)  +  3 CO2(g)  +  3 H2O(l)

Anhydrous PrCl3 can be made by dehydration of the hydrate either by slowly heating to 400 °C with 4-6 equivalents of ammonium chloride under high vacuum, , or by heating with an excess of thionyl chloride for four hours ,. The anhydrous halide may alternatively be prepared from praseodymium metal and hydrogen chloride. It is usually purified by high temperature sublimation under high vacuum.

Uses
PrCl3 has been used to increase the activity of 6O11 catalysts, which can be used for the oxidation of methane to ethene. This process is becoming an important route to ethene for the manufacture of polyethylene (a common plastic). There are no major uses for praseodymium(III) chloride, though it can be used as a starting point for the preparation of other praseodymium salts.

Precautions
Praseodymium compounds are of low to moderate toxicity, although their toxicity has not been investigated in detail. Wear gloves and goggles.