THE TIME EVOLUTION OF THE HARD AND SOFT ACIDS AND BASES THEORY - Theoretical and Computational Research in the 21st Century - page 16

degree of covalency in the bonding and form a mutually stabilized com-
plex.
If a hard metal is combined with a soft ligand, the metal does not read-
ily accept the electron density being offered by the ligand, and therefore
the resulting complex is less stable. In practice, the HSAB principle has
proved remarkably useful in indicating favorable combinations of ligands
and metal ions. A close inspection of the Tables (1.1) and (1.2) will pro-
vide with many useful examples that demonstrate the general validity of
this approach. It is clear from Pearson's hard-soft classifi cation scheme
that there are some inherent properties of the acids and the bases that gov-
ern their relative strength toward a chemical reaction. Later, one very im-
portant inherent property of the acids and the bases that governs the rela-
tive strength of an acid toward a base and vice versa in a chemical reaction
was identifi ed as hardness [24(a), (b)].
We shall discuss the concept of hardness in the following section [vide
infra].
1.2.6 THE HSAB CLASSIFICATION IN TERMS OF THE
CONCEPT OF ELECTRONEGATIVITY
The concept of electronegativity provides a measure of the intrinsic
strength of an acid or base [3, 25, 26]. A strong Lewis acid is a good
electron acceptor and has high electronegativity/low chemical potential. A
weak Lewis acid has a lower electronegativity than a strong Lewis acid,
but it has a higher electronegativity than a Lewis base. A strong Lewis
base readily donates electrons and has a lower electronegativity than a
weak Lewis base. These relations are summarized by Ayers [3] as follows:
χ(strong acid) > χ(weak acid) > χ(weak base) > χ(strong base) > 0 (4)
The perfect electron donor has χ = 0. One can reify the electron-accept-
ing abilities of real molecules by imagining how they would react with a
perfect electron donor.
The HSAB concept was basically an empirical one. The dependence of
relative strength of acids and bases on each other was not explained in this
principle. There are also a number of extrinsic infl uences, and the concept
of hard and soft concerns itself with these infl uences, such as the reference
base used to determine the strength of acids and the reference acid used to