The coordination geometry of water in some salt hydrates

The orientations of water molecules next to metal ions in salt hydrates are useful indications of the orientations which may be expected for water molecules which are next to metal ions in aqueous solution. A survey of the data for the salt hydrates shows that a water molecule may often be tetrahedral (i.e., coordinated to the metal ion, accepting a hydrogen bond from another water molecule, and donating two hydrogen bonds) if it is next to a +1 ion, a+2 ion, or a+3 ion of the rare earth series, although apparently not for smaller +3 ions.