Cobalt(II) iodide or cobaltous iodide are the inorganic compounds with the formula CoI<sub>2</sub> and the hexahydrate CoI<sub>2</sub>(H<sub>2</sub>O)<sub>6</sub>. These salts are the principal iodides of cobalt.
Cobalt(II) iodide crystallizes in two polymorphs, the ñ- and ò-forms. The ñ-polymorph consists of black hexagonal crystals, which turn dark green when exposed to air. Under a vacuum at 500 ðC, samples of ñ-CoI<sub>2</sub> sublime, yielding the ò-polymorph as a yellow crystals. ò-CoI<sub>2</sub> also readily absorbs moisture from the air, converting into green hydrate. At 400 ðC, ò-CoI<sub>2</sub> reverts to the ñ-form. The anhydrous salts adopt the cadmium halide structures.
The hexaaquo salt consists of separated [Co(H<sub>2</sub>O)<sub>6</sub>]<sup>2+</sup> and iodide ions as verified crystallographically.
Cobalt(II) iodide is prepared by treating cobalt powder with gaseous hydrogen iodide The hydrated form CoI<sub>2</sub>.6H<sub>2</sub>O can be prepared by the reaction of cobalt(II) oxide (or related cobalt compounds) with hydroiodic acid.
Anhydrous cobalt(II) iodide is sometimes used to test for the presence of water in various solvents.
Cobalt(II) iodide is used as a catalyst, e.g. in carbonylations. It catalyzes the reaction of diketene with Grignard reagents, useful for the synthesis of terpenoids