THE TIME EVOLUTION OF THE HARD AND SOFT ACIDS AND BASES THEORY - Theoretical and Computational Research in the 21st Century - page 7

on a single atom or delocalized over several atoms in a molecule. Usually,
only closed shells are considered to be involved in acid-base-type interac-
tions, and usually a pair of electrons is involved in the process.
Although the strengths of acids and bases can be adequately described
in terms of the relative dissociation constant (p K ) values of the acids and
the bases within the Lowry-Brønsted acid-base theory, [7] there was no
single scale of Lewis acidity and basicity, and also there was no whis-
per of measuring the acid-base strength and the ease of the reaction. The
importance of the Lewis acid-base concept is that almost all inorganic
compounds can be viewed as acid-base adducts or complexes, A:B. This
is also true for organic compounds. Thus, the Lewis acid-base concept [8]
covers almost the major area of chemical reactions and offers a grand uni-
fi ed model of that dissects any molecule mentally into parts: acid and base.
Moreover, there are numerous reasons that the Lewis theory has not
replaced the Lowry-Brønsted model [7] for protic acids. Even when we
try to classify the strength of Lewis acids qualitatively comparing their
reactions with a common base, we fi nd that it is impossible to come up
with a single scale that encompasses all possible Lewis acids. Some Lewis
acids (e.g., Al 3+ , BF 3 ) act very much like H + when reacted with a base.
Thus, we could use reactions with Al 3+ to set up a scale of p K a's very
similar to that using H + . But, other Lewis acids, for example, Hg 2+ , give
an entirely different series of base strengths if the same procedure is fol-
lowed for them. The origin of this strange behavior is believed to lie in the
fact that the origin of protic acid strength is primarily electrostatic. But,
when all Lewis acids are considered, a pure electrostatic model cannot be
put forward. Many species react with bases using a signifi cant amount of
covalent bonding, and these cannot be directly related to the others that are
compatible with an electrostatic model.
The scientists noticed forever that mother nature always shows a path
to understand the naturally occurring facts and to reach at the core of the
phenomenon. As for example, in nature, metal ions such as Mg 2+ or Al 3+
are observed as oxide and carbonate ores, whereas Cd 2+ , Hg 2+ and Pt 2+ ap-
pear as sulphides and arsenide ores. It is also known that certain ligands
formed their most stable complexes with metal ions such asAl 3+ , Ti 4+ , and
Co 3+ , whereas others formed stable complexes with Ag + , Hg 2+ , and Pt 2+ .
The prediction of the reactivity of a generalized acid-base reaction and
the stability of the acid-base adduct were very important. But at that time,