title: Cobalt(II) iodide
text: # Cobalt(II) iodide
  
  Cobalt(II) iodide or cobaltous iodide are the inorganic compounds with the formula $CoI_2$ and the hexahydrate $CoI_2(H_2O)_6.$ 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 $CoI_2.6H_2O$ 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 $α-CoI_2$ sublime, yielding the β-polymorph as a yellow crystals. $β-CoI_2$ also readily absorbs moisture from the air, converting into green hydrate. At 400 °C, $β-CoI_2$ reverts to the α-form.
  
  ## Structures
  
  The anhydrous salts adopt the cadmium halide structures.
  
  The hexaaquo salt consists of separated $[Co(H_2O)_6]^{2+}$ and iodide ions as verified crystallographically.
  
  ## 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
  
  