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Thallium triiodide

Thallium triiodide
Other names thallous triiodide
CAS number 60488-29-1
Molecular formula TlI3
Molar mass 585.097 g/mol
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and 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+ + 2I→ Tl+ + I2

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

Tl3+ + 2e→ Tl+  ; Ero = 1.252
I2 + 2e→ 2I  ; Ero = 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 TlIII 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, Ia−Ib−Ic, ions in the different compounds are shown below:

compound Ia−Ib (pm) Ib - Ic (pm) angle (°)
TlI3 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

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. 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Thallium_triiodide". A list of authors is available in Wikipedia.
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