Calcium Hexammoniate, Ca(NH₃)₆

Metallic calcium dissolves in liquid ammonia, forming a blue solution. On passing ammonia over metallic calcium at 15° - 20° C. heat is evolved, and a bronze-coloured solid, which takes fire in air, is left. Moissan gave to it the formula Ca(NH₃)₄.

Kraus studied the vapour pressure of the system calcium: ammonia. Two liquid phases in contact with one another may be obtained. At -33° C. the dilute phase did not exceed 1/10 gram-atom per litre, whilst the concentrated phase had a metallic reflection. The form of the vapour pressure curve indicated the presence of only one compound and that had a composition Ca(NH₃)₆.

By heating under reduced pressure, Botolfsen observed explosive decomposition which he ascribed to an unstable ammoniate of composition Ca₂(NH₃), and he suggested the existence of a series of ammonium compounds.

The following values for the dissociation pressures of the hexammoniate have been found: -

Temperature, ° C	-11	12	44
Pressure in mm. Mercury	17.5	50	307

The heat of formation calculated from these dissociation pressures is 10.32 Cal.

Calcium hexammoniate exhibits metallic electrical conduction, as also does the liquid from which it is precipitated. It is probable that ammonia is combined with the metal in the same way as in ammoniated salts or solvated ions, and that in solution a free positive ion, Ca(NH₃)₆²⁺, such as is present to some extent when a calcium salt is dissolved in ammonia, and two negative electrons are formed. Gibson and Argo consider that the colour of the solution is due to the combination of the solvent with these electrons.

Acetylene reacts with calcium ammonium at -70° C., giving a compound to which the formula CaC₂.C₂H₂.4NH₃ has been ascribed.