Thallium triiodide

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

Additional recommended knowledge

How to quickly check pipettes?

How to ensure accurate weighing results every day?

Safe Weighing Range Ensures Accurate Results

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.

Structure and preparation

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.

General references

• WebElements
• Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.