Template:Chembox new Thallium triiodide is a chemical compound of thallium and iodine with formula TlI₃. Unlike the other thallium trihalides, which contain thallium(III), TlI₃ is a thallium(I) compound and contains the triiodide ion, I₃⁻. An appreciation as to why Tl⁺ is not oxidised to Tl³⁺ in the reaction:

Tl³⁺ + 2I⁻→ Tl⁺ + I₂

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

Tl³⁺ + 2e→ Tl⁺  ; E_r^o = 1.252

I₂ + 2e→ 2I⁻  ; E_r^o = 0.5355;

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

It should be noted that 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 Tl^III with excess I⁻ forming the TlI₄⁻ ion.

TlI₃ is formulated Tl⁺ I₃⁻, and has a similar structure to NH₄I₃, CsI₃ and RbI₃.^[1] The triiodide ion in TlI₃ is nearly linear but is asymmetric with one iodine−iodine bond longer than the other. For comparison the dimensions of the tri-iodide, I_a−I_b−I_c, ions in the different compounds are shown below:

TlI₃ can be prepared by the evaporation of stoichiometric quantities of TlI and I₂ in concentrated aqueous HI, or by reacting TlI with I₂ in ethanol.