Thallium triiodide
Thallium triiodide, more precisely thallium triiodide is a chemical compound of thallium and iodine with empirical formula. Unlike the other thallium trihalides, which contain thallium, is actually a thallium salt containing thallium cations and triiodide anions, and thus has the chemical formula. It is a black crystalline solid.
An appreciation as to why is not oxidised to 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 to .
Using standard electrode potentials in this way must be done with caution as factors such as complex formation and solvation may affect the reaction. is no exception as it is possible to stabilise thallium with excess forming the tetraiodothallate ion .
Structure and preparation
is formulated, and has a similar structure to [ammonium triiodide|], [caesium triiodide|] and [Rubidium triiodide|]. The triiodide ion in is nearly linear but is asymmetric with one iodine-iodine bond longer than the other. For comparison the dimensions of the triiodide, Ia-Ib-Ic, ions in the different compounds are shown below:| compound | Ia-Ib | Ib-Ic | angle |
| 306.3 | 282.6 | 177.90 | |
| 305.1 | 283.3 | 178.11 | |
| 303.8 | 284.2 | 178.00 | |
| 311.4 | 279.7 | 178.55 |
can be prepared by the evaporation of stoichiometric quantities of thallium(I) iodide and iodine in concentrated aqueous hydriodic acid, or by reacting TlI with iodine in ethanol.