Theoretical Basis of Hardness and Softness

Theoretical Basis of Hardness and Softness

Theoretical Basis of Hardness and Softness

Theoretical Basis of Hardness and Softness

In order to explain the stability of complexes, different theories have been proposed, formed as a result of interactions between hard and soft elements. Some important theories are discussed below-

1. π-Bonding theory

Mulliken and Chatt proposed this hypothesis to explain soft-soft interaction on the basis of π-bonding. Soft acids have a low oxidation state and have an large number of d-electrons. Thus, they have a strong tendency to form π-bonds with soft bases which are also good π–bonding ligands. The polarization of soft acids and soft bases even prefers π-bonding.

2. Electronegativity and Hardness & softness

Elements with high electronegativity are hard and elements with low electronegativity are soft. The relation between electronegativity and hardness helps to explain the fact that CF3 is considerably harder than CH3 and BF3 is harder than BH3.

For example lithium has low electronegativity, Li+ has relatively high tendency to attract electrons toward itself, and therefore, it has high electronegativity. This is because of extremely high second ionisation potential. Consequently, Li+ is hard acid. On the other hand, transition metal ions in low oxidation state such as Cu+ , Hg+ , Ag+ , Cd+2 etc have relatively high second ionisation energies and therefore have low value of electronegativity. Therefore, they are considered as soft acids. The same way we can consider hard and soft bases.

3. Electrostatic Interactions

This hypothesis states that the interaction of hard acids and hard bases leads to the formation of an ionic bond. Hard-hard interactions are thus purely ionic or electrostatic. The electrostatic force of attraction between two oppositely charged ions is inversely proportional to the internuclear distance. The internuclear distance will be less in the case of smaller ions. Therefore, the electrostatic attraction between two ions will be more prominent and consequently the resulting compound formed by hard acid and hard base will be highly stable.

4. Polarizing power and Polarizability

Covalent bond is formed by the interaction of soft acids and soft bases because the soft acids and soft bases have large size. The polarization effects are, therefore important to explain their interactions. Soft acids are generally transition metal ions having six or more d-electrons. The d-sub-shell are easily polarized. Therefore, the complexes formed by soft acids and soft bases have covalent bonding and are stable.