HSIP Hard-Soft Interaction

Applications of the Hard-Soft Interaction Principle (HSIP)... 

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. 

The primary reason for discussing the hard-soft interaction principle at this time is because of its usefulness in dealing with solubility and solvation. Certainly, the principle like dissolves like has been known for a very long time. We wiU mention here only a few aspects of the HSIP and its relationship to solubihty. As an example, we can consider that NaCl is essentially insoluble in nitrobenzene (/t = 4.27D). Even though nitrobenzene is quite polar, it can not solvate ions hke Na or Cl because of the size of the molecules. It is polar, but depends on both the quantity of charge separated and the distance of separation. Since nitrobenzene is a large molecule, its size causes the dipole moment to be large, but it also limits the ability of the molecules to solvate small ions. 

In other words, OH is a much stronger base than SH . The orbital holding the unshared pair of electrons on the oxygen atom is smaller than the one holding the electron pair on the sulfur atom. Therefore, H+ interacts more strongly with OH- than it does with SH . The HSIP does not say that hard acids will not bond to soft bases. Rather, it says that bonding between hard acids and hard bases is more effective than between hard acids and soft bases. A similar statement can also be made for bonding as a result of the interaction between soft acids and soft bases. 

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