Praseodymium diiodide

Praseodymium diiodide is a chemical compound with the empirical formula of PrI₂, consisting of praseodymium and iodine. It is an electride, with the ionic formula of Pr³⁺(I⁻)₂e⁻,^[2] and therefore not a true praseodymium(II) compound.

Praseodymium diiodide

Identifiers
CAS Number	65530-47-4
3D model (JSmol)	Interactive image
InChI InChI=1S/2I.Pr Key: MOULFFIZRHFNND-UHFFFAOYSA-N
SMILES [I].[I].[Pr]
Properties
Chemical formula	I2Pr
Molar mass	394.71660 g·mol−1
Appearance	bronze solid[1][2]
Melting point	758 °C[1][2]
Related compounds
Other cations	Lanthanum(II) iodide Cerium(II) iodide Neodymium(II) iodide
Related compounds	Praseodymium(III) iodide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Preparation

Praseodymium diiodide can be obtained by reacting praseodymium(III) iodide with metallic praseodymium at 800 °C to 900 °C in an inert atmosphere:^[3]

Pr + 2 PrI₃ → 3 PrI₂

It can also be obtained by reacting praseodymium with mercury(II) iodide where praseodymium displaces mercury:^[3]

Pr + HgI₂ → PrI₂ + Hg

Praseodymium diiodide was first obtained by John D. Corbett in 1961.^[4]

Properties

Praseodymium diiodide is an opaque, bronze-coloured solid with a metallic lustre that is soluble in water.^[3] The lustre and very high conductivity can be explained by the formulation {Pr^III,2I⁻,e⁻}, with one electron per metal centre delocalised in a conduction band.^[2]

The compound is extremely hygroscopic, and can only be stored and handled under carefully dried inert gas or under a high vacuum.^{[citation needed]} In air it converts into hydrates by absorbing moisture, but these are unstable and more or less rapidly transform into oxide iodides with the evolution of hydrogen:^{[citation needed]}

2 PrI₂ + 2 H₂O → 2 PrOI + H₂↑ + 2 HI

With water, these processes take place much faster.^[3]

Praseodymium diiodide has five crystal structures, namely the MoSi₂ structure, the hexagonal MoS₂ structure, the trigonal MoS₂ structure, the cadmium chloride structure and the spinel structure.^[5] Praseodymium diiodide with the cadmium chloride structure belongs to the trigonal crystal system, with the space group R3m (No. 166), lattice parameters a = 426.5 pm and c = 2247,1 pm; however, the spinel structure of praseodymium diiodide is cubic,^[6] with space group F43 (No. 216), and lattice parameter a = 1239.9 pm.^[7]