Cobalt(II) iodide explained

Cobalt(II) iodide or cobaltous iodide are the inorganic compounds with the formula CoI₂ and the hexahydrate CoI₂(H₂O)₆. These salts are the principal iodides of cobalt.

Synthesis

Cobalt(II) iodide is prepared by treating cobalt powder with gaseous hydrogen iodide. The hydrated form CoI₂.6H₂O can be prepared by the reaction of cobalt(II) oxide (or related cobalt compounds) with hydroiodic acid.

Cobalt(II) iodide crystallizes in two polymorphs, the α- and β-forms. The α-polymorph consists of black hexagonal crystals, which turn dark green when exposed to air. Under a vacuum at 500 °C, samples of α-CoI₂ sublime, yielding the β-polymorph as a yellow crystals. β-CoI₂ also readily absorbs moisture from the air, converting into green hydrate. At 400 °C, β-CoI₂ reverts to the α-form.^[1]

Structures

The anhydrous salts adopt the cadmium halide structures.

The hexaaquo salt consists of separated [Co(H₂O)₆]²⁺ and iodide ions as verified crystallographically.^{[2] [3]}

Reactions and applications

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

Notes and References

1. O. Glemser "Cobalt, Nickel" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 1518.
2. “Structure Cristalline et Expansion Thermique de L’Iodure de Nickel Hexahydrate“ (Crystal structure and thermal expansion of nickel(II) iodide hexahydrate) Louër, Michele; Grandjean, Daniel; Weigel, Dominique Journal of Solid State Chemistry (1973), 7(2), 222-8.
3. "The crystal structure of the crystalline hydrates of transition metal salts. The structure of CoI₂·6H₂O" Shchukarev, S. A.; Stroganov, E. V.; Andreev, S. N.; Purvinskii, O. F. Zhurnal Strukturnoi Khimii 1963, vol. 4, pp. 63-6.