THE TIME EVOLUTION OF THE HARD AND SOFT ACIDS AND BASES THEORY - Theoretical and Computational Research in the 21st Century - page 52

the thiocyanate anion, where more than one coordinating sites of different
hardness are present. Hence, the HOMO-LUMO concept is not sufficient
to explain the chemical reactivity for such compounds. To explain the
chemical reactivity for such molecules, formed by ambidentate ligands, a
new concept was introduced by da Silva et al. [74, 75] known as “frontier
effective-for-reaction molecular orbital” (FERMO). This concept, using
an intuitive statement based on the HOMO-LUMO approach to the hard-
ness and softness, presented the FERMO-LUMO gaps. In the FERMO
concept, MO composition and shape are taken into account to identify the
MO that will actually be involved in a given reaction. A molecule could
have as many FERMOs as it has reactions sites, and it could be the HOMO
or any other FMO. da Silva et al. [74, 75] applied the FERMO concept to
the HSAB principle [9] for ambidentate ligands and correctly described
the soft and hard sites in case of the systems they studied.
1.2.22 PROOFS FOR HSAB PRINCIPLE
The Pearson's HSAB concept was basically formulated to explain the di-
rection of an acid-base double-exchange reaction . The proof of HSAB
principle requires a realization of the HSAB concept or some idea to dem-
onstrate its feasibility, or a demonstration in principle, whose purpose is to
verify that some concept or theory has the potential of being used. A proof
of concept is usually small and may or may not be complete. However,
it is not easy to theoretically establish the hard and soft acids and bases
principle, Chattaraj et al. [76] offered two proofs for HSAB principle with
a restriction “among potential partners of a given electronegativity, hard
likes hard and soft likes soft.”
Here, it is important to mention that using a heterolytic dissociative
version of Pauling bond energy equation, Patra et al. [77] also made some
attempts to provide qualitative and quantitative proof for the HSAB prin-
ciple of Pearson.
1.3 CONCLUSION
The HSAB Theory is an Indispensable Theoretical Construct of Concep-
tual Chemistry. The Pearson's HSAB concept was basically formulated
to explain the direction of an acid-base double-exchange reaction. The
concept and efforts of its quantitative measurement evolved with time.