Thallium triiodide

Thallium triiodide is a chemical compound of <a href="/facts/Thallium/v35pGMJB">thallium</a> and <a href="/facts/Iodine/JlfMecMf">iodine</a> with formula TlI3. Unlike the other <a href="/facts/Thallium_halides/LN55ac3w">thallium trihalides</a>, which contain thallium(III), TlI3 is a thallium(I) salt and contains the <a href="/facts/Triiodide/fNgW9WEr">triiodide</a> ion, I−3.
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 <a href="/facts/Reduction_potential/PqpNUgSl">standard reduction potentials</a> of the half cells which are:

Tl3+ + 2 e− → Tl+; Er° = 1.252
I2 + 2 e− → 2 I−; Er° = 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 (TlI−4) ion (<a href="/facts/Isoelectronic/SbzuaI05">isoelectronic</a> with the <a href="/facts/Potassium_tetraiodomercurate(II)/YkB2d5FU">tetraiodomercurate</a> anion and with lead(IV) iodide).

Thallium triiodide open-in-new

Thallium triiodide