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Mixed crystals and double salts between metal(II) salt hydrates

The salts with predominantly ionic crystal lattices are considered as ionic coordination compounds. The main factors determining their structures are the sizes of ions and their charges, the energy of metal-ligand bonds and the energy determined by the positional symmetry of the ligands. The preference for coordination of the ligands towards the metal ions is discussed both on the basis of Pearson's "hard-soft Lewis acids and bases" concept and in terms of the "Lewis base strength" of the anions formulated by Brown i.e. the valences available for metal-ligand bonds. The crystal field stabilization energy is considered as a factor which opposes some external influences and therefore limits the possibilities for deformation of the coordination polyhedra. The mixed crystal and double salt formation are discussed on the basis of these principles. A model for isomorphous and isodimorphous mixing is developed. This model allows the calculation of the distribution coefficients of the components between the mixed crystals and the solution in the cases of isomorphous mixing. In the case of isodimorphous mixing the free energies of phase transitions can be estimated. Using solubility diagram data for salt-salt-water system, relationships between the crystal structures of the salt components and the type of solubility diagrams are shown. The double salt formation is caused by differences in the first coordination sphere of the two metal ions. For this to occur they must differ considerably in size and charge or in the "hard-soft" factor. When polyatomic anions are involved, the "Lewis base strength" of the anions plays an important role. The zero crystal field stabilization energy of the metal ions is an additional factor which favours the double salt formation.