The affinity of metals and their ions toward active groups can be predicted to some extent on the basis of the principle of hard and soft acids and bases (HSAB theory). This theory explains empirically the basicity of metal its affinity for a particular ligand and the bond strength between the metal and the ligand attached to a biologically active molecule.
The assumptions of HSAB theory are
- If there is a bond between two different atoms or groups of atoms one plays the role of an acid and the other that of a base.
- Electrons hold the bonded atoms together and
- Electron mobility decides the softness or hardness of the acid and bases.
An acid or base’s polarizability plays a role in its reactivity. Tiny, lightweight, and non-polarizable hard acids and bases. Weak acids and bases are heavier, and the distribution of electrons is more dispersed. Strong acids react with strong bases preferentially, and soft acids react with soft bases preferentially.