Manganese(II) iodide is the chemical compound composed of manganese and iodide with the formula MnI<sub>2</sub>(H<sub>2</sub>O)<sub>n</sub>. The tetrahydrate is a pink solid while the anhydrous derivative is beige. Both forms feature octahedral Mn centers. Unlike MnCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub> and MnBr<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub> which are cis, MnI<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub> is trans.
Anhydrous MnI<sub>2</sub> is prepared from the elements:
The tetrahydrate can be prepared by treating manganese(II) carbonate with hydriodic acid. The anhydrous form can be produced from it by dehydration in a vacuum.
Samples turn brown in air under the influence of light as a result of the oxidation of the iodide ion to iodine. It has a trigonal crystal structure of the cadmium iodide type (polytype 2H) with the space group Pm1 (space group no. 164). It dissolves in water and decomposes. The tetrahydrate has a monoclinic crystal structure with the space group P2<sub>1</sub>/c (No. 14).
It is often used in the lighting industry.