THE TIME EVOLUTION OF THE HARD AND SOFT ACIDS AND BASES THEORY - Theoretical and Computational Research in the 21st Century - page 31

FIGURE 1.4 Correlation of the HOMO-LUMO gap, the hardness, and the
electronegativity with the hard and soft behavior of chemical species.
Pearson [9(c)] further correlated the phenomenon that “unsaturation
increases softness” with the HOMO-LUMO energy gap that usually de-
fi nes the lowest-energy absorption band. Soft acids and bases should ab-
sorb light closer to the visible light than hard acids and bases. The chromo-
phoric groups always correspond to relatively high energy HOMOs and
low energy LUMOs.
Thereafter, in 1988, Pearson [52] has evaluated methods to rank the
various Lewis acids and bases according to their local hardness derived
from hemolytic bond dissociation data.
The details of the concepts of the electronegativity and hardness, their
origin, and their physcochemical behavior will be explained in the later
section.
1.2.11 THEORETICAL DEDUCTION OF THE HSAB PRINCIPLE
In their landmark paper, Parr and Pearson [46], based on the assumption
that the energy of a chemical system is quadratic function of the number
of electrons, suggested a theoretical formalism of the HSAB principle. In
order to do so, they proposed a two-step mechanism for the formation of
the A:B adduct from its constituents: acid (A) and base (B).
(i) The first step involves shift of some charge, ∆N from B to A.
(ii) The second step involves the formation of the actual chemical bond
between the acid, A and the base, B.