Hard-soft add-base

The Hard-Soft-Add-Base (HSAB) theory was developed by Pearson in 1963. According to this theory, Lewis acids and Lewis bases are divided into two groups on one hand hard acids and bases, which are usually small, weakly polarizable species with highly localised charges, and on the other hand soft acids and bases which are large, polarizable species with delocalised charges. A selection of Lewis acids, ordered according to their hardness in aqueous solution is presented in Table 1.3. 

Synthesis of Human GIP from Protected GIP by the Hard-Soft Add-Base Procedure " ... 

Interaction Energy for Hard-Soft Add-Base Exchange Reactions Involving Polyatomic Molecules... 

Pearson s Hard-Soft-Acid-Base (HSAB) priciple is that hard add-base combinations form readily and are generally ionic compounds. The other group of stable compounds and complex ions involves the interaction between soft acid and soft bases. For these, the bonding is primarily covalent with interpenetrating orbitals. The combinations hard acid with soft base, or vice versa, have little stability. 

In our original work, we used an ionic-covalent model to interpret the E and C parameters. It has been suggested that our E and C parameters are a quantitative manifestation of the hard-soft model. "Softness (or hardness") can be considered (67) as a measure of the ratio of the tendency of a spedes to undergo covalent interaction to the tendency of the species to undergo electrostatic interaction. The relative "softness or hardness is depicted in the C/E ratio. The ratios for the acids and bases can be calculated from the data in Tables 3 and 4. If the ratio C/E is comparatively large, the add or base would be classified as type B or soft. Inasmuch as the relative ratios of C/E tells the relative importance of the two effects for various donors and acceptors, we agree that the hardness or softness discussed in the HSAB model is given by this ratio. 

McDaniel and coworkers have presented a graphical means of portraying some of the ideas discussed in this chapter. For the reaction of hard and soft adds and bases... 

Pearson " designated the class (a) metal ions of Ahrland, Chatt, and Davies as hard acids and the class (b) ions as soft acids. Bases are also classified as hard or soft on the basis of polarizability the halide ions range from F , a very hard base, through less hard Cl and Br to I , a soft base. Reactions are more favorable for hard-hard and soft-soft interactions than for a mix of hard and soft reactants. Hard adds and bases are relatively small, compact, and nonpolarizable soft acids and bases are larger and more polarizable. The hard acids include cations with a large positive charge (3-t or larger) or those whose... 

Idealized Hard Species Add A B Base Hard—Hard Interaction Idealized Soft Species Acid A B Base Soft-Soft Interaction... 

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