Thallium triiodide

Thallium triiodide
IUPAC name thallium(I) (triiodide)
Other names thallous triiodide
Identifiers
CAS number	60488-29-1 Y
	Properties
Molecular formula	TlI3
Molar mass	585.097 g/mol
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

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⁺ + 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^o = 1.252

I₂ + 2 e− → 2 I⁻ ; E_r^o = 0.5355;

The favoured reaction is therefore the reduction of Tl³⁺ (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 TlI₄⁻ ion (isoelectronic with the tetraiodomercurate anion and with lead(IV) iodide).

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, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419. 

Footnotes

1. ↑ Die Kristallstrukturen von Rubidiumtriiodid und Thalliumtriiodid Tebbe K.F., Georgy U Acta Crystallographica,C (1986) 42, 1675 doi:10.1107/S0108270186090972