Coordination chemistry as known for transition metals with conventional ligands (say NH₃) is lacking for both alkali metals (M⁺) and alkaline earth metals (M²⁺) due to their very weak coordinating (or complexing) ability. These metals ions were thought to be inadequate for complexation due to their small charge to size ratio. The lack of physico-chemical prosperities such as U.V. visible and magnetic properties which help to detect even weak metal – ligand interaction, was a signification reason for this delayed exploration of complexation ability of Groups 1 & II metal cations.

Application of X-ray crystallography to the study of biologically important molecules, such as antibiotics, showed that some of these molecules are involved in the complexation of M⁺(Na⁺ , K⁺) or M²⁺ (Ca²⁺, Mg²⁺) ion. For example the antibiotic nonactin functions by transporting potassium ions out of cells. This molecule prefers to bind K⁺ ions over Na⁺ ions , i.e., it is not only capable of binding a weakly coordinating K⁺ but is also highly selective.

These studies revealed insight into the underlying parameters of complexation and selectivity. The stability of these natural complexes is attributed to a large number of donor sites present in a single molecule which can act (donate) simultaneously, i.e., wrap around the cation. The selectivity or preference of a ligand for particular cation is determined by the size of the ‘cavity’ generated by each ligand when it wraps around the metal cation.

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