Aluminium Iodide

Here are the facts. Don't skim.

Chemical Compound

Aluminium Iodide

Ball-and-stick model of the aluminium iodide dimer, Al₂I₆. It’s a clumsy, bridged structure born of necessity.

Names
Preferred IUPAC name Aluminium iodide
Other names Aluminium(III) iodide Aluminum iodide Aluminium triiodide Aluminum triiodide

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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is Y N ?)
Infobox references

Aluminium iodide is an inorganic compound with the straightforward composition AlI₃. Don't overcomplicate it. It also exists in various hydrated forms, which arise when the anhydrous material is carelessly exposed to moisture. [4] The hexahydrate, for instance, is what you get from a reaction between metallic aluminum or aluminum hydroxide with hydroiodic acid.

Like its more common relatives, aluminium chloride and bromide, AlI₃ is a strong Lewis acid. This is a technical way of saying it's desperate for electrons and will greedily absorb water directly from the atmosphere—a trait of chemical neediness. In practical terms, it's employed as a reagent for the surgical scission of certain C-O and N-O bonds. It's a chemical scalpel, cleaving aryl ethers and deoxygenating epoxides with a precision that borders on violent. [5]

Preparation

The synthesis of aluminium iodide is brutishly simple. It's formed by the direct, exothermic reaction of aluminium and iodine, [6] or by subjecting aluminium metal to the action of HI.

The direct synthesis is a spectacle of chemical impatience. When a few drops of water are added to a homogenized mixture of aluminum powder and powdered iodine, there's a brief, deceptive pause—an induction period—before the system erupts. A vigorous, exergonic reaction occurs, unleashing intense colored vapors.

• 2Al(s) + 3I₂(s) → 2AlI₃(s)

The dramatic purple clouds are due to iodine, which evaporates as a consequence of the rapidly increasing temperature. The brown smoke is likely an adduct of the reaction product with any excess iodine, a messy but predictable outcome of such a forceful combination.

Structure

In its solid state, AlI₃ doesn't like to be alone. It is dimeric, consisting of Al₂I₆, a structural arrangement similar to that of AlBr₃. [3] The architecture of both the monomeric and dimeric forms has been thoroughly characterized in the gas phase. [7]

The monomer, AlI₃, is trigonal planar, a flat and symmetrical molecule with an Al-I bond length of 2.448(6) Å. The bridged dimer, Al₂I₆, observed at 430 K, is a more complex affair, resembling its cousins Al₂Cl₆ and Al₂Br₆. It features two types of bonds: terminal Al−I bonds with a length of 2.456(6) Å and bridging Al−I bonds stretched to 2.670(8) Å. This dimer is described as "floppy," possessing an equilibrium geometry of D₂h. It's a molecule held together by necessity, not elegance.

Aluminium(I) iodide

The name "aluminium iodide" is almost universally assumed to describe the triiodide or its dimer. This is a common, if limited, understanding. In fact, a monoiodide, AlI, also plays a role in the Al–I system. However, this compound is unstable at room temperature, prone to disproportionation, collapsing back into the more stable triiodide: [8]

• 3 AlI → AlI₃ + 2 Al

This is a chemical inevitability, where the less stable form breaks down, leaving behind its more robust relative and a residue of elemental aluminium, like a shed skin.

An illustrative, and far more stable, derivative of aluminium monoiodide is the cyclic adduct it forms with triethylamine, Al₄I₄(NEt₃)₄. Here, the otherwise fleeting AlI is captured and stabilized within a larger molecular framework, a chemical curiosity proving that even the unstable can be contained.