Thallium triiodide
Encyclopedia
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:
can be gained by considering the standard reduction potentials
of the half cells which are:
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.
− , and has a similar structure to NH4I3, CsI3 and RbI3. 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:
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.
Thallium
Thallium is a chemical element with the symbol Tl and atomic number 81. This soft gray poor metal resembles tin but discolors when exposed to air. The two chemists William Crookes and Claude-Auguste Lamy discovered thallium independently in 1861 by the newly developed method of flame spectroscopy...
and iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
with formula TlI3. Unlike the other thallium trihalides, which contain thallium(III), TlI3 is a thallium(I) compound and contains the triiodide
Triiodide
In chemistry, triiodide can have several meanings. Triiodide primarily refers to the triiodide ion, I3−, a polyatomic anion composed of three iodine atoms. For some chemical compounds, triiodide indicates a salt of the named cation with the triiodide anion. Examples include sodium 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
Reduction potential
Reduction potential is a measure of the tendency of a chemical species to acquire electrons and thereby be reduced. Reduction potential is measured in volts , or millivolts...
of the half cells which are:
- Tl3+ + 2
→ Tl+ ; Ero = 1.252 - I2 + 2
→ 2 I− ; Ero = 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
Structure and preparation
TlI3 is formulated Tl+ I3| compound | Ia |
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.