Hard soft acid-base exchange reactions principle

Translation of the Hard and Soft Acids and Bases principle into a readily applicable formal expression has never been achieved. Huheey suggested, that the driving force to the HSAB rule is a strong hard-hard interaction only, in a set of an acid/base exchange reaction [18]. Nalewajski presented a thorough discussion of that topic [57] and summarized older concepts. Soft-soft interaction is predominantly covalent, whereas hard-hard interaction occurs through ionic forces. The classical formula for energy (Eq. 17) explains why a soft-soft interaction is favored over a soft-hard one the favorable hard-hard situation cannot be accounted for by Eq. 17. 

Chattaraj, P. K. Ayers, P. W. Melin, J. Further Links Between the Maximum Hardness Principle and the Hard/Soft Acid/Base Principle Insights from Hard/Soft Exchange Reactions. Phys. Chem. Chem. Phys., 2007,9, 3853-3856. 

Which catalyst should be chosen for a given reaction will depend upon chemical, steric, and mechanistic factors. The application of Pearson s soft and hard acid-base (SHAB) principle has often proved a valuable qualitative guide as to suitable surface sites for a particular reactant. In fact, certain solids actually owe their catalytic power to attached Bronsted or Lewis acid and base groups as exemplified by weak acid ion exchange resins (Sect. 2.3), alumina (Sect. 3.2), and sometimes charcoals. Steric aspects can be con-... 

From a mechanistic point of view, ion exchange reactions follow the hard and soft acids and bases (HSAB) principle. Materials containing the relatively soft imidazolium ion are therefore particularly efficient ion exchange materials for soft anionic species. For this reason, the measured distribution coefficients between the solid and liquid phases are often particularly high with soft anions and follow Hofmeister selectivity [142]. 

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