Thallium triiodide

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

Thallium triiodide is a chemical compound of thallium and iodine with formula ${\ce {TlI3}}$ . Unlike the other thallium trihalides, which contain thallium(III), ${\ce {TlI3}}$ 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

$TlI 3$ is formulated $Tl + I 3 -$ , and has a similar structure to NH₄I₃, CsI₃ and RbI₃.^[1] The triiodide ion in $TlI 3$ 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:

compound	I_a−I_b (pm)	I_b - I_c (pm)	angle (°)
$TlI 3$	306.3	282.6	177.9
RbI₃	305.1	283.3	178.11
CsI₃	303.8	284.2	178.00
NH₄I₃	311.4	279.7	178.55

$TlI 3$ can be prepared by the evaporation of stoichiometric quantities of $TlI$ and $I 2$ in concentrated aqueous $HI$ , or by reacting $TlI$ with $I 2$ in ethanol.

General references

WebElements
Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.

Footnotes

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

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

[1]