Electronic polarizability with acid-base chemistry

Hardness and softness as chemical concepts were presaged in the literature as early as 1952, in a paper by Mulliken [138], but did not become widely used till they were popularized by Pearson in 1963 [139]. In the simplest terms, the hardness of a species, atom, ion or molecule, is a qualitative indication of how polarizable it is, i.e. how much its electron cloud is distorted in an electric field. The adjectives hard and soft were said to have been suggested by D.H. Busch [140], but they appear in Mulliken s paper [138], p. 819, where they characterize the response to spatial separation of the energy of acid-base complexes. The analogy with the conventional use of these words to denote resistance to deformation by mechanical force is clear, and independent extension, by more than one chemist, to the concept of electronic resistance, is no surprise. The hard/soft concept proved useful, particularly in rationalizing acid-base chemistry [141]. Thus a proton, which cannot be distorted in an electric field since it has no electron cloud (we ignore the possibility of nuclear distortion) is a very hard acid, and tends to react with hard bases. Examples of soft bases are those in which sulfur electron pairs provide the basicity, since sulfur is a big fluffy atom, and such bases tend to react with soft acids. Perhaps because it was originally qualitative, the hard-soft acid-base (HSAB) idea met with skepticism from at least one quarter Dewar (of semiempirical fame) dismissed it as a mystical distinction between different kinds of acids and bases [142]. For a brief review of Pearson s contributions to the concept, which has been extended beyond strict conventional acid-base reactions, see [143],... 

To rationalize observations such as these, Pearson presented the concept of hard and soft acids and bases (HSABs), designating polarizable acids and bases as soft and nonpolarizable acids and bases as hard. Much of the hard-soft distinction depends on polarizability, the degree to which a molecule or ion is distorted by interaction with other molecules or ions. Electrons in polarizable molecules can be attracted or repelled by charges on other molecules, forming slightly polar species that can then interact with the other molecules. The HSAB concept is a nseful gnide to explain acid-base chemistry and other chemical phenomena. Pearson stated, Hard acids prefer to bind to hard bases, and... 

We have already alluded to one of the most useful and pervasive principles in aU of chemistry, that being the hard-soft interaction principle (HSIP). This principle relates to many areas, but it is most directly applicable to interactions in which there is electron pair donation and acceptance (Lewis acid-base interactions). The terms hard and soft relate essentially to the polarizability of the interacting species. For example, 1 has a large size, so its electron cloud is much more distortable than that of F . Likewise, Hg2+ is a large metal ion having a low charge, while Be " " is a very small ion. The result is that Hg is considered to be a soft Lewis acid while Be is considered to be a hard Lewis acid. As a result of these characteristics, Hg + interacts preferentially with 1 rather than F , while Be " " interacts preferentially with F . The hard-soft interaction principle indicates that species of similar electronic character (hard or soft) interact best. It does not say that hard Lewis acids will not interact with soft Lewis bases, but the interaction is more favorable when the acid and base are similar in hard-soft character. 

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