Aqua species of Mg²⁺‏, Ca²⁺‏, Sr²⁺‏ and Ba²⁺‏

   Using ¹⁷O-labelled water, it is possible to confirm the hydration number in [Mg(H₂O)₆]²⁺‏ by use of ¹⁷O NMR spectroscopy, but for the aqua ions of Ca²⁺‏, Sr²⁺  and Ba²⁺‏, exchange between coordinated water and solvent is too fast on the NMR timescale for such studies. An octahedral coordination sphere has been established for [Mg(H₂O)₆]²⁺‏ in a number of crystalline salts, and also for [Ca(H₂O)₆]²⁺‏. The [Mg(H₂O)₆]²⁺‏ ion dissociates to some extent in aqueous solution, but hydrated cations of the later metals are not appreciably ionized and solutions of their salts derived from strong acids are neutral.