Hard-soft, acid-bases absolute hardness table

For electron donors, a simple attempt at classifying hard and soft bases is illustrated Table 6.3. Such classification schemes reveal, not suipris-ingly, that hard and soft are not absolute, but gradually varying qualities. Pearson (1963) has proposed, in generalizing hardness and softness properties for Lewis acids and bases for many kinds of systems, the so-called HSAB (hard and soft acid-base) rules. 

The hard character of the BF3 affinity scale has been confirmed theoretically. The theoretical absolute hardness of BF3, p = 9.7 eV, is fairly high compared with other molecular Lewis acids (see Table 1.17). The hardness of the interaction of BF3 with NH3, NMc3 and CO has been studied [70] through the local Hard-Soft Acid-Base principle. 

In this equation, r) the absolute hardness, is one-half the difference between /, the ionization potential, and A, the electron affinity. The softness, a, is the reciprocal of T]. Values of t) for some molecules and ions are given in Table 8.4. Note that the proton, which is involved in all Brdnsted acid-base reactions, is the hardest acid listed, with t — c (it has no ionization potential). The above equation cannot be applied to anions, because electron affinities cannot be measured for them. Instead, the assumption is made that t) for an anion X is the same as that for the radical Other methods are also needed to apply the treatment to polyatomic... 

Finally, let us not forget that there is a single, simple frame of understanding which correctly, albeit qualitatively, predicts the energetic order of mercury carbodiimide and mercury cyanamide, and it is based on chemical ideas, namely Pearson s concept of hard and soft acids and bases soft prefers soft and hard prefers hard. The absolute softness of Hg + (7.7 eV, see Table 2.3) lies between those of Pb and Ag+ such that a cyanamide anion and not a carbodiimide anion will be the preferred bonding partner for Hg +, just as for Pb + and Ag+. I cannot refrain from noting that I find this quite remarkable. 

Based on the research of Klopman, and Parr and Pearson (Klopman 1968 Parr and Pearson 1983), Pearson described the absolute hardness (ri) quantitatively as being proportional to the difference between I (ionization potential) and A (electron affinity) of the species (Pearson 1988). Absolute softness is defined as t). The absolute electronegativity (x) and the absolute hardness (Tj) are applied quantitatively to any given acid-base reaction. Table 3.10 presents x and Tj values for some representative metal ions. 

The relationship 5.20 is shown in Figure 5.13. It is difficult to assess the relative importance of model and experimental errors in these correlations. However, it is clear that the Ai7(Bl2)-Av(I—CN) correlation is less family dependent than the pAlBi2- i (I I) and p7fBicN-Av(I—CN) relationships shown in Figures 5.12 and 5.10. Hence the relationships 5.19 and 5.20 may support the use of Av(I—CN) as a spectroscopic scale of soft affinity. Indeed, diiodine is the archetype of soft Lewis acids in the Pearson classification since it has a very low absolute hardness (rj = 3.4 eV). Moreover, Av(I—CN) values obey the HSAB principle (soft acids prefer soft bases) since they decrease with the absolute hardness of the donor atom (in a given column of the periodic table), as shown in Table 5.28. 

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