THE TIME EVOLUTION OF THE HARD AND SOFT ACIDS AND BASES THEORY - Theoretical and Computational Research in the 21st Century - page 23

The products E A E B and C A C B were roughly related to trends in the elec-
trostatic and covalent contributions to the enthalpy.
The theoretical basis of the Drago's equation has been of some inter-
est. Klopman [24(b)] has shown that Eq. (10) is consistent with his per-
turbation theoretical approach to intermolecular interactions with the E
parameters corresponding to a charge-controlled contribution and the C
parameters corresponding to frontier-controlled, or covalent binding.
A theoretical support for the four-parameter equation was given by
Marks and Drago on the basis of charge-transfer formulation [35]. They
[35] also gave support to relating the E parameters to ionic interactions and
the C parameters to covalent. Drago and Kabler [36] opined that “the idea
that some substances are just stronger acids or bases than others with re-
gard to both covalent and electrostatic interactions are invariably ignored
in HSAB principle,” and suggested to employ the terms “large C property”
or “frontier controlled” for soft and “large E property” or “charge con-
trolled” for hard. Substances can then be considered to have both a large C
and a large E relative to some other substance. Large size, low ionization
energy, and other properties leading to softness' often contribute to a large
C , whereas small size and high charge, etc., contribute to a large E.
Pearson [37] as a reply to the comment of Drago and Kabler [36] pro-
posed a possible quantitative statement of HSAB as follows:

log K = S A S B + σ A σ B (11)

This is a typical four-parameter equation, two independent parameters
for both the acid and base. It is most closely related to the equation pro-
posed by Edwards and Pearson [20].

log K = αE n + βH (12)

with α = σ A , E n = σ B , β = S A , and H = S B .
Pearson [37] again correlated Eq. (11) to the Drago's four-parameter
equation and found that the most consistent interpretation would be C A =
σ A , C B = σ B , E A = S A , and E B = S B .
Pearson commented that the hardness and softness parameters are
quite different from Drago and Kabler's [36] mysterious suggestion that C
is softness and E is hardness.
As a reply to the Pearson's comment, Drago [38] stated that the hard-
soft model has its basis in arguments that are related to the strength of