Thallium triiodide

Thallium(I) triiodide
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Template:Longitem	Template:Chem2
Molar mass	Template:Chem molar mass
Appearance	black crystalline solid
NFPA 704 (fire diamond)	Template:NFPA 704 diamond
Template:Longitem	Template:Ubl
Template:Longitem	Phosphorus heptabromide (Template:Chem2)

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Thallium triiodide, more precisely thallium(I) triiodide is a chemical compound of thallium and iodine with empirical formula Template:Chem2. Unlike the other thallium trihalides, which contain thallium(III), Template:Chem2 is actually a thallium(I) salt containing thallium(I) cations Template:Chem2 and triiodide anions Template:Chem2, and thus has the chemical formula Template:Chem2. It is a black crystalline solid.

An appreciation as to why Template:Chem2 is not oxidised to Template:Chem2 in the reaction:

Template:Chem2

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

Template:Chem2; E_r° = 1.252

Template:Chem2; E_r° = 0.5355

The favoured reaction is therefore the reduction of Template:Chem2 to Template:Chem2 (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. Template:Chem2 is no exception as it is possible to stabilise thallium(III) with excess Template:Chem2 forming the tetraiodothallate(III) ion Template:Chem2 (isoelectronic with the tetraiodomercurate anion Template:Chem2 and with lead(IV) iodide Template:Chem2).

Structure and preparation

Template:Chem2 is formulated Template:Chem2, and has a similar structure to [[ammonium triiodide|Template:Chem2]], [[caesium triiodide|Template:Chem2]] and [[Rubidium triiodide|Template:Chem2]].^[1] The triiodide ion in Template:Chem2 is nearly linear but is asymmetric with one iodine–iodine bond longer than the other. For comparison the dimensions of the triiodide, 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 (°)
Template:Chem2	306.3	282.6	177.90
Template:Chem2	305.1	283.3	178.11
Template:Chem2	303.8	284.2	178.00
Template:Chem2	311.4	279.7	178.55

Template:Chem2 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.