Ion coordinated hydrates

Water molecules can coordinate to metal cations, and does so strongly to the smaller ions such as Na+, Li+, Ca2+ and Mg2+. For transition metals, this interaction is particularly strong. These aquo complexes can exhibit significant stability with water loss occurring well above the boiling point of liquid water, as a result of ion dipole interactions with the metal centre. For 'hard' (non-polarisable) metal ions with a high charge to radius ratio the water is often labile (in fast exchange with its surroundings) whereas softer metals are more inert. While metal aquo complexes exist in all state of matter, recognition of a material as an ion-coordinated hydrate implies its isolation as a crystalline solid, usually with a well-defined number of water molecules per formula unit. There are three known crystalline solids of formula CrCl3.6H2O, for example. Ion conductivity measurements show that their formulae should be written [Cr(H2O)6]Cl3, [Cr(H2O)5Cl]Cl2.H2O and [Cr(H2O)4Cl2]Cl.2H2O.1 The aquo ions are enclosed in the square brackets and exist in both the solid state and in solution. The water after the 'dot' is water of crystallization and only forms part of the crystalline hydrate.

1. A. F. Wells, Structural Inorganic Chemistry, Clarendon, Oxford, 3rd ed, 1962.