The Physical Basis of the Hard/soft Acid/base Principle

The dependence of molecular properties on the chemical hardness is explored. As the chemical hardness of a molecule increases, its size and polarizability typically decrease and its charge and electronegativity typically increase. On the basis of these properties, the interaction energy between hard and soft acids and bases is quantified, and the physical basis of the global and local hard/soft acid/base (HSAB) principles is elucidated.