Praseodymium(III) iodide

• Heating praseodymium and iodine in an inert atmosphere produces praseodymium(III) iodide:[1]

${\displaystyle {\mathsf {2Pr+3I_{2}\ \xrightarrow {T} \ 2PrI_{3}}}}$

• It can also be obtained by heating praseodymium with mercury(II) iodide:[5]

${\displaystyle {\mathsf {2Pr+3HgI_{2}\ \xrightarrow {T} \ 2PrI_{3}+3Hg}}}$

Praseodymium(III) iodide forms green crystals, which are soluble in water.[4]

Praseodymium(III) iodide forms orthorhombic crystals which are hygroscopic.[1] It crystallizes in the PuBr₃ type[5][6] with space group Cmcm (No. 63) with a = 4.3281(6) Å, b = 14.003(6) Å and c = 9.988(3) Å.[7]

PrI₃ forms compounds with hydrazine, like I₃Pr·3N₂H₄·4H₂O which has pale yellow crystals and soluble in methanol, slightly soluble in water, and insoluble in benzene, d_{20 °C} = 2.986 g/cm³.[8]

PrI₃ forms compounds with urea, like I₃Pr·5CO(NH₂)₂ which has pale green crystals.[9]

PrI₃ forms compounds with thiourea, like I₃Pr·2CS(NH₂)₂·9H₂O which is a green crystal with d = 2.27 g/cm³.[4][10]

Praseodymium(III) iodide forms a nonahydrate, PrI₃·9H₂O. It can be obtained by dissolving praseodymium(III) oxide in concentrated aqueous hydroiodic acid:

Pr₂O₃ + 6 HI + 15H₂O → 2 PrI₃·9H₂O

It adopts the same structure as other light rare earth iodides (La–Ho) and contains a triangular tricapped prismatic nonaaqua ion [Pr(OH₂)₉]³⁺ and iodide counterions.[11]

Praseodymium(III) iodide reacts with praseodymium metal at elevated temperatures to form praseodymium diiodide:[12]

2 PrI₃ + Pr → 3 PrI₂