THE TIME EVOLUTION OF THE HARD AND SOFT ACIDS AND BASES THEORY - Theoretical and Computational Research in the 21st Century - page 36

1.2.14 QUANTIFICATION OF HSAB PRINCIPLE

Pearson [55], in order to refine HSAB principle that originally refers to
the bond dissociation energy (∆E) of the generalized acid-base reactions,
Eq. (1) stated that the misinterpretation often made of the HSAB principle
is the use of a single parameter (hardness or its inverse, softness) to de-
termine the acid-base interaction, and this single-parameter assumption
leads to the failure of the HSAB principle describing many chemical reac-
tions. Moreover, the linear free-energy relationships that have been used
to describe and predict the strengths of acid and base interactions require
two parameters rather than one parameter for each the acid and the base,
suggesting that at least one extrinsic influence on acid-base strength is in
evidence. Thoughtful examination of these parameters and their interrela-
tionships has suggested the existence of the HSAB principle [56].
Pearson [9(b), 57] in an attempt to head off this erroneous perception,
assumed that the other entire factors, except softness, that determine the
bond energy in a acid-base interaction may be considered as intrinsic
strength (S) that depends on (i) the charge and (ii) the size of the interact-
ing acid and base. He [57] assumed that the acid and base strength is singly
ordered, and the acid-base interaction reaction (Eq. (1)) was given by him
as follows:

S A S B = -∆E (45)

As the reaction depends not only on the intrinsic strength but also on
the softness parameter (σ), then the actual interaction reaction represented
by Pearson is as follows:

S A S B + σ A σ B = -∆E (46)

He [57] also pointed out that σ is positive for soft species and σ is nega-
tive for hard species, and this attempt gives extra stabilization of HSAB
principle.
Equation (46) reveals that this equation resembles to the well-known
four-parameter equation, Eq. (1.10), of Drago et al. [34].
Based on the experimental data, Pearson opined that for an exchange
reaction of the type, Eq. (47), the only requirement is the relative hard-
ness of the reactants to predict theoretically the preferred direction of the
chemical reaction: