Thallium triiodide, more precisely thallium(I) triiodide is a chemical compound of thallium and iodine with empirical formula . Unlike the other thallium trihalides, which contain thallium(III), is actually a thallium(I) salt containing thallium(I) cations and triiodide anions , and thus has the chemical formula . It is a black crystalline solid.
Redox properties Traditionally, the reason why is not oxidised to in the reaction:
is ascribed to the standard reduction potentials of notional half-cells which are:
;Er° = 1.252 ;Er° = 0.5355
The favoured reaction is therefore the reduction of to (1.252 > 0.5355). Using standard electrode potentials in this way must be done with caution, if considering species beyond neutral aqueous solution. Electrode potentials vary (and vary differently) with pH, with solvent, and with any other ligands present.
In fact, thallium triiodide reacts in alcoholic base to give thallium(III) species, and adds additional iodide ions to give the tetraiodothallate(III) ion (isoelectronic with tetraiodomercurate, ). These properties may extend in methanol to neutral pH: the ultraviolet spectrum suggests that in such solutions, TlI3 ionizes to Tl[TlI4]3.
Structure and preparation is formulated , and has a similar structure to , and . 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 (pm) Ib–Ic (pm) 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 (TlI) and iodine in concentrated aqueous hydriodic acid, or by reacting TlI with iodine in ethanol.
References
General references: WebElements