Chemical Hardness Principles Reloaded

DEFINITION CRl (a) A molecule (ion) is defined as hard if it has an energetic broad energetic gap between the orbitals HOMO and LUMO. (b) One defines a molecule (ion) as being soft if it has a narrow energetic gap between the orbitals HOMO and LUMO. 

FIGURE 3.9 Delocalization and polarization in the chemical reaction process of a hard molecule with a soft one, having the same electronegativity. 

Therefore, these defining properties are reasonable in the light of the so far presented concepts of chemical reactivity. The weak acids and bases have properties, which ensure them that the HOMO levels are relatively high in energy and the LUMO levels are relatively at low energies. The strong acids and bases have opposite properties. 

Regarding the polarization, the quantum theory proves that in the presence of an external electric field, the excited states are mixed with the fundamental one, so that the resulted energy to be as low as possible. For the weak acids and bases, i.e., characterized by a narrow energetic band LUMO-HOMO = I-A, the polarizability is high because all the excited states in spectrum are enlarged near the fundamental state. Therefore, also a principle regarding the reactivity (as an interaction) between the acids and bases can be expressed  

PRINCIPLE CRI (of Hard and Soft Acids and Bases - HSAB) Hard acids prefer coordination with the hard bases, and the soft acids prefer coordination with the soft bases. 

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