Complex Formation by Inorganic Ions

The formation of complexes MN(m-n)+ from Mm+ and Nn- in aqueous solution is discussed in terms of simple electrostatic forces. The entropy changes are shown to depend on similar factors to those which determine the enthalpy changes. In cases where the entropy changes for a series of complexes with different Mm+ and constant Nn- are either linearly related to the enthalpy changes, or can be neglected, the free energy of formation of the complex from the ions may be predicted in unknown cases. The relations commonly used for plotting the free energy of formation of the complex from the ions as functions of a number of physicochemical quantities are shown to arise fortuitously as a consequence of the restricted range of ionic radii provided by Nature. In some cases deviations from expected behaviour show the influence of structural changes on the ease of complex formation.