Thallium triiodide

Thallium triiodide, more precisely thallium(I) triiodide is a chemical compound of thallium and iodine with empirical formula TlI₃. Unlike the other thallium trihalides, which contain thallium(III), TlI₃ is actually a thallium(I) salt containing thallium(I) cations Tl⁺ and triiodide anions [I₃]⁻, and thus has the chemical formula Tl⁺[I₃]⁻. It is a black crystalline solid.

An appreciation as to why Tl⁺ is not oxidised to Tl³⁺ in the reaction:

Tl³⁺ + 2 I⁻ → Tl⁺ + I₂

can be gained by considering the standard reduction potentials of the half-cells which are:

Tl³⁺ + 2 e⁻ → Tl⁺; E_r° = 1.252

I₂ + 2 e⁻ → 2 I⁻; E_r° = 0.5355

The favoured reaction is therefore the reduction of Tl³⁺ to Tl⁺ (1.252 > 0.5355).

Using standard electrode potentials in this way must be done with caution as factors such as complex formation and solvation may affect the reaction. TlI₃ is no exception as it is possible to stabilise thallium(III) with excess I⁻ forming the tetraiodothallate(III) ion [TlI₄]⁻ (isoelectronic with the tetraiodomercurate anion [HgI₄]²⁻ and with lead(IV) iodide PbI₄).