THE TIME EVOLUTION OF THE HARD AND SOFT ACIDS AND BASES THEORY - Theoretical and Computational Research in the 21st Century - page 6

In this work, we shall follow the locus of the several attempts that were
made to the refi ne and improve the qualitative HSAB principle growing to
the pinnacle of present glory. However, we may point out that quantifi ca-
tion of HSAB principle is still at large.
Attempts were made to develop theoretical models of HSAB principle
in terms of the density functional theory [10, 11]. But, although many
qualitative descriptors relating to HSAB principle could be quantifi ed in
terms of density functional theory, no satisfactory theoretical quantitative
model that can unify the various facets of the principle into one and same
principle is still at large. We are still after a grand theoretical model that
can smoothly distinguish between the soft and hard character of acids and
bases and their bonding. We propose to have a serious relook at the HSAB
principle with an open mind. In order to fi nd out a way to quantify the
qualitative principle and place them on a sound quantitative platform, fur-
ther attempts are required. This is just the main goal of the present work.

1.2 SUMMARY OF THE EARLY WORKS

1.2.1 HISTORY OF LEWIS ACID-BASE THEORY

Lewis acid-base concept [8] is one of the most useful schemes in concep-
tual chemistry. Based on this concept, almost entire chemistry was refor-
mulated in terms of these so-called Lewis compounds. It is worth noting
that the Lewis base definition seems to have consumed the Lowry -Brøn-
sted theory [7(a), (b)] in its fold. A Lewis acid is an atom, ion, or molecule
that has a tendency to act as an electron acceptor, whereas a Lewis base is
an atom, molecule, or ion that tends to act as an electron donor. An acid
and base interaction may be viewed as the lone pair electron donation
from the base (B) to the acid (A) resulting in the formation of a coordinate
covalent bond between an acid and a base creating an “acid-base adduct”
(A:B).

A + B → A:B (1)

Thus, in brief, the Lewis theory is basically a theory of electron accountan-
cy and magic numbers. Commonly, an acid-base interaction includes any
degree of electron transfer of nonbonding, bonding, or antibonding elec-
trons from the base to an unfilled nonbonding, bonding, or antibonding
electronic state on the acid. The electronic states involved may be localized