Hard soft acid-base exchange reactions

Iwamoto et al. (54) studied the activity of a series of metalion exchanged zeolites for the water-gas shift reaction. The lower water-gas shift activity of the acidic cations was explained in terms of hard-soft acid/base properties. In this model, carbon monoxide, which is a soft base, interacts more strongly with soft acid sites. The adsorption of CO is generally considered to be the rate controlling step in the water-gas shift reaction. Cations of lower acidity are generally softer acids and as such adsorb CO more readily. This would lead to higher surface concentrations of CO, thereby increasing the water-gas shift acitivity of the sample. 

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-... 

Thus we can understand easily enough that the reaction between bromine and ethylene giving dibromoethane is exothermic—it replaces one n bond (C=C) with two a bonds (C—Br) at the expense of a weak a bond (Br—Br). However, it is not always obvious how strong the bonds will be when one molecule combines with another to form a single new molecule, or what happens to the energy if we exchange parts of one molecule with parts of another. A useful addition to understanding this sort of problem has been Pearson s concept of hard and soft acids and bases (HSAB). 

Here, the exchange reactions in gas-phase between two acid-base complexes have been considered and the reaction energy has assumed as a measure of the stabilization of hard-hard and soft-soft adducts in comparison to the corresponding hard-soft and soft-hard counterparts. The used general scheme for the acid-base reactions during which a base (X), bonded to the hard acid (H+), removes a hard base (OH") from a soft acid (HO ) is ... 

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]. 

Suggest a mechanism and a hard and soft Lewis acids and bases (HSAB) rationale for the following ligand exchange (metathesis) reaction ... 

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