Thallium triiodide

Thallium triiodide, more precisely thallium(I) triiodide is a chemical compound of thallium and iodine with empirical formula TlI₃. Unlike the other thallium trihalides, which contain thallium(III), TlI₃ is actually a thallium(I) salt containing thallium(I) cations Tl⁺ and triiodide anions [I₃]⁻, and thus has the chemical formula Tl⁺[I₃]⁻. It is a black crystalline solid.

Thallium(I) triiodide

Thallium triiodide
Names
IUPAC name Thallium(I) triiodide
Other names thallous triiodide
Identifiers
CAS Number	60488-29-1 Y 13453-37-7
3D model (JSmol)	Interactive image
ChemSpider	14888331
EC Number	236-627-3
PubChem CID	20233350
CompTox Dashboard (EPA)	DTXSID70928661
InChI InChI=1S/I3.Tl/c1-3-2;/q-1;+1 Key: CFEVJLSKLTWQJS-UHFFFAOYSA-N
SMILES [Tl+].I[I-]I
Properties
Chemical formula	TlI3
Molar mass	585.09 g·mol−1
Appearance	black crystalline solid
Hazards
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H300, H330, H373, H411
NFPA 704 (fire diamond)	4
Related compounds
Other cations	Ammonium triiodide Rubidium triiodide Caesium triiodide
Related compounds	Phosphorus heptabromide ([PBr4]+[Br3]−)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

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 of the half-cells which are:

Tl³⁺ + 2 e⁻ → Tl⁺; E_r° = 1.252

I₂ + 2 e⁻ → 2 I⁻; E_r° = 0.5355

The favoured reaction is therefore the reduction of Tl³⁺ to Tl⁺ (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 tetraiodothallate(III) ion [TlI₄]⁻ (isoelectronic with the tetraiodomercurate anion [HgI₄]²⁻ and with lead(IV) iodide PbI₄).

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 triiodide, I_a–I_b–I_c, ions in the different compounds are shown below:

compound	Ia–Ib (pm)	Ib–Ic (pm)	angle (°)
TlI3	306.3	282.6	177.90
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 thallium(I) iodide (TlI) and iodine in concentrated aqueous hydriodic acid, or by reacting TlI with iodine in ethanol.