THE TIME EVOLUTION OF THE HARD AND SOFT ACIDS AND BASES THEORY - Theoretical and Computational Research in the 21st Century - page 18

THE PEARSON-PAULING PARADOX

It is interesting to point out that Pearson [28] has called attention to an in-
teresting anomaly between the HSAB rule and Pauling's original methods
of defining electronegativity [29]. In Pauling method, the ionic resonance
energy is considered as proportional to the square of the difference in elec-
tronegativity of the constituent atoms. This implies that the most stabiliza-
tion occurs when the bonds are formed between the constituents furthest
apart in electronegativity. Thus, the ionic resonance energy suggests writ-
ing the following reaction, in the forward direction as follows:

CsI + LiF→LiI + CsF (5)

s-s h-h h-s s-h

Here, s and h stands for harder and softer, respectively.
But, experimentally it is found that the reverse reaction, which follows
the HSAB rule, is energetically favorable.

LiI + CsF → CsI + LiF(∆H ≈ −63kJ/mol) (6)

This paradox is popularized in the text book of chemistry as Pearson-
Pauling paradox.

1.2.7 LOCAL HSAB PRINCIPLE

Ayers and Parr [30] recently pointed out the ignored contribution from the
Pearson's work [9(a)] is the local HSAB principle. The local HSAB prin-
ciple [30] was stated as follows:
An ambidentate reagent with a hard reactive site and a soft reactive
site tends to react with hard molecules at the hard site and soft molecules
at the soft site.
This proposition introduced the concept of “local hardness”—the idea
that different atoms and functional groups within a molecule have differ-
ent hardnesses.
We shall discuss the local hardness in the following section (vide in-
fra).