Hard-Soft Acid-Base Theory and Nucleophilicity

Hard-Soft Acid-Base Theory and Nucleophilicity 

Pearson and Edwards noted that some substrates in Sn2 reactions seem more susceptible to the Bronsted basicity of a nucleophile, while other substrates seem more susceptible to its polarizability. Pearson proposed that bases be divided into two categories soft (polarizable) and hard (nonpolarizable). On the basis of equilibrium data, Pearson concluded that H, Li, Na, Mg, and Ca are hard acids, while Cu, Ag, Hg, I, Br, I2, and Br2 are soft. Pearson noted that hard acids show greater association with hard bases, while soft acids prefer soft bases. One explanation for this behavior is the ionic/covalent bond description, in which it is assumed that hard acids and hard bases are attracted primarily through ionic interactions, while soft acids and soft bases associate through covalent bonds. In addition, the two 

For the data plotted, the correlation coefficient is only 0.7, but a general trend is evident. 

Data are for reaction of the two ions with CH3I in methanol solution. See Pearson, R. G. Sobel, H. Songstad, J. /. Am. Chem. Soc. 1968, 90, 319. 

Pearson appbed the concept of hard and soft acids and bases to nucleo-philicity in the 2 reaction by using the concept of absolute hardness, rj  

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