It is an electride, with the ionic formula of Pr3+(I−)2e−,[2] and therefore not a true praseodymium(II) compound.

[4] Praseodymium diiodide is an opaque, bronze-coloured solid with a metallic lustre that is soluble in water.

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

[5] Praseodymium diiodide with the cadmium chloride structure belongs to the trigonal crystal system, with the space group R3m (No.