HSAB Principle and Maximum Hardness Index

However, EE-eneigy based relation pemiits the immediate recovery of the Hard and Soft Acids and Bases (HSAB) principle (Pearson, 1985 Parr Pearson, 1983 Klopman, 1968 Chandrakumar Pal, 2002a Chattaraj Maiti, 2003), an important conceptual principle to treat the molecular binding and reactive processes. Writing the energy variation at transfer under the following forms (Chattaraj Parr, 1993)  

Therefore, the condition to achieve the optimum transfer results to be  

The above relations were inferred for the general case, for which the form of the chemical potential (respectively the electronegativity with changed sign) is not specified. 

From now on, the molecular systems are recognized as hard and soft acids and bases (HSAB), in the sense that each molecule can be seen as hard-hard, soft-soft, hard-soft or soft-hard bonding combinations between acids and bases. The associate HSAB principle of chemical reactivity was formulated as well, providing that hard acids prefer hard bases and soft acids prefer soft bases (Drago Kabler, 1972 Pearson, 1972 Chattaraj etal., 1991)  

Despite the qualitative character (Nalewajski, 1984 Chattaraj Sengupta, 1996 Komorowski et al., 1996 Ponti, 2000 Chandrakiunar Pal, 2002a-c Chattaraj Maiti, 2003 Torrent-Sucarrat et al., 2005 Putz et al., 2004.) of the HSAB principle, an appropriate quantum index to smoothly distinguish between the soft and hard character of acids, bases, and their bonding, would switch HSAB towards a quantitative theory. 

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