Thallic iodide, TlI₃

Thallic iodide, TlI₃, may be prepared by digesting thallous iodide with an alcoholic solution of iodine and allowing the solution to evaporate over sulphuric acid. It forms large, dark, orthorhombic crystals (a:b:c = 0.6828:1:1.1217), isomorphous with rubidium and csesium tri-iodides. It might therefore be concluded that thallic iodide is a per-iodide of thallous iodide and essentially a thallous compound. According to Abegg and Maitland, it is a tautomeric substance, behaving as a thallous or as a thallic compound according to circumstances. In its sparing solubility in water and organic solvents it resembles the thallous halides, while it resembles the other thallic halides in forming complex salts with other iodides. In aqueous solution these double salts yield the stable complex ion TlI₄'.