Zinc iodide

Zinc iodide is the inorganic compound with the formula ZnI₂. It exists both in anhydrous form and as a dihydrate. Both are white and readily absorb water from the atmosphere. It has no major application.

Preparation

It can be prepared by the direct reaction of zinc and iodine in water or refluxing ether:
Absent a solvent, the elements do not combine directly at room temperature.

Structure as solid, gas, and in solution

The structure of solid ZnI₂ is unusual relative to the dichloride. While zinc centers are tetrahedrally coordinated, as in ZnCl₂, groups of four of these tetrahedra share three vertices to form “super-tetrahedra” of composition, which are linked by their vertices to form a three-dimensional structure. These "super-tetrahedra" are similar to the P₄O₁₀ structure.
Molecular ZnI₂ is linear as predicted by VSEPR theory with a Zn-I bond length of 238 pm.
In aqueous solution the following have been detected: Zn₆²⁺, ⁺, tetrahedral ZnI₂₂, ZnI₃⁻, and ZnI₄²⁻.

Applications

Zinc iodide is often used as an x-ray opaque penetrant in industrial radiography to improve the contrast between the damage and intact composite.
United States patent 4,109,065 describes a rechargeable aqueous zinc-halogen cell that includes an aqueous electrolytic solution containing a zinc salt selected from the class consisting of zinc bromide, zinc iodide, and mixtures thereof, in both positive and negative electrode compartments.
In combination with osmium tetroxide, ZnI₂ is used as a stain in electron microscopy.
As a Lewis acid, zinc iodide catalyzes for the conversion of methanol to triptane and hexamethylbenzene.
It can be used as a topical antiseptic.