Aluminium iodide

Invariably, the name refers to a compound of the composition AlI3, formed by the reaction of aluminium and iodine[4] or the action of HI on Al metal.

Like the related chloride and bromide, AlI3 is a strong Lewis acid and will absorb water from the atmosphere.

[3] The structure of monomeric and dimeric forms have been characterized in the gas phase.

The name "aluminium iodide" is widely assumed to describe the triiodide or its dimer.

In fact, a monoiodide also enjoys a role in the Al–I system, although the compound AlI is unstable at room temperature relative to the triiodide:[7] An illustrative derivative of aluminium monoiodide is the cyclic adduct formed with triethylamine, Al4I4(NEt3)4.

Ball and stick model of aluminium iodide dimer
Ball and stick model of aluminium iodide dimer
Experiment showing a direct synthesis of aluminum iodide. Few drops of water are added to a homogenised mixture of aluminum powder and powdered iodine. After short time (an induction period) a vigorous reaction occurs followed by emission of intense colored vapors. The purple vapours are due to evaporation of iodine as a consequence of increased temperature of the system, and the brown ones are probably due to smoke of an adduct of the reaction product with excess of iodine. The exergonic reaction 2Al(s) + 3I 2 (s) → 2AlI 3 (s) is at the origin of the phenomenon observed.