The Hard-Soft Interaction Principle HSIP

As we have seen, the Lewis theory of acid-base interactions based on electron pair donation and acceptance applies to many types of species. As a result, the electronic theory of acids and bases pervades the whole of chemistry. Because the formation of metal complexes represents one type of Lewis acid-base interaction, it was in that area that evidence of the principle that species of similar electronic character interact best was first noted. As early as the 1950s, Ahrland, Chatt, and Davies had classified metals as belonging to class A if they formed more stable complexes with the first element in the periodic group or to class B if they formed more stable complexes with the heavier elements in that group. This means that metals are classified as A or B based on the electronic character of the donor atom they prefer to bond to. The donor strength of the ligands is determined by the stability of the complexes they form with metals. This behavior is summarized in the following table. 

Cr3 + and Co3 + belong to dass A because they form more stable complexes with oxygen as the donor atom than when sulfur is the donor atom. On the other hand, Ag+ and Pt2+ belong to dass B because they form more stable complexes with P or S as the donor atom than with N or O as the donor atom. 

A modification of the HSAB approach was first explained by C. K. Jorgensen in connection with the stability of a cobalt complex. Under normal circumstances, Co3+ is a hard Lewis acid. However, when 

The hard-soft acid-base principle is not restricted to the usual types of acid-base reactions. It is a guiding principle that for all types of interactions species of similar electronic character interact best. We have already seen some applications (such as the relative strength of HF and HI) of this principle, which we will continue to call HSAB, but we now consider a number of other types of applications. 

A clear effect of hydrogen bonding is afforded by considering the interaction of alcohols with acetonitrile, CH3CN, and trimethylamine, (CH3)3N. The dipole moments of these molecules are 3.44D 

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 applications of the hard-soft interaction principle are numerous. For example, if we consider the potential interaction of H with either H2O or 1 , where does go  

We find that H+, being a hard acid (electron pair acceptor), interacts preferentially with a pair of electrons in a small orbital on the oxygen atom rather than the pair of electrons in a large orbital on I . Accordingly, HI is completely ionized in dilute aqueous solutions as a result of the protons being transferred to H2O. However, it must be emphasized that such a proton transfer from HI to H2O is energetically unfavorable in the gas phase. The extremely high heat of solvation of H makes this reaction take place in solutions, so the process is not quite as simple as shown earlier. 

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. 

The solubility of NaCl in water and alcohols also shows an interesting trend and allows us to see the effects of solvent properties. The relevant data are shown in Table 5.3. As the size of the solvent molecules increases and the dielectric constant decreases, the solubihty of NaCl decreases. The size and character of the alkyl group becomes dominant over that of the polar OH group. Accordingly, the solubihty of ionic solids such as NaCl decreases with increasing size of the alkyl group. 

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

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