THE TIME EVOLUTION OF THE HARD AND SOFT ACIDS AND BASES THEORY - Theoretical and Computational Research in the 21st Century - page 17

determine the strength of bases. This particular extrinsic infl uence is called
hardness, and its compliment and softness were coined by Pearson.
Reed, using a simple orbital model, recently explored the structural
origin of hard-soft behavior in atomic acids and bases [27].
He discussed the physical basis of hard-soft behavior in terms of the
set of atomic orbitals and the properties that have been associated with
them. These properties are atomic size, orbital energy, energy gap, abso-
lute hardness, relaxation energy, atomic charge, absolute electronegativity,
polarizability, ionization energy, and electron affi nity.
Among the relevant sets of atomic orbitals are the frontier orbitals, the
responding electrons, and the whole valence shell. Associated with each
of these structures are properties that have over the years been linked to
hard-soft behavior.
The size of the species, its frontier orbital energy, and its frontier or-
bital energy gap are among the relevant properties of the frontier orbit-
als. The importance of frontier electrons and frontier orbitals is implied in
the Lewis defi nition; thus, frontier orbitals were among the fi rst electronic
structures to be identifi ed with acid-base behavior. The size of the occu-
pied frontier orbital dictates the size of an atom. A reasonable and simple
indicator of the relative size of an atom or ion is provided by the average
distance of this electron from its nucleus. This may be obtained for a hy-
drogenic atomic orbital and is found to be inversely related to ε r . Pearson
[9] has also associated the gap in the energies of the occupied frontier
orbital and the unoccupied frontier orbital, Δ ε r , which is the difference in
the energy of the frontier electron and the lowest-energy virtual electron,
with the absolute hardness.
Reed [27] pointed out that there have been several interpretations of
hard-soft behavior, and among them one interpretation has considered
hard and soft to involve two different properties: one of which dominates
in a hard acid or base and the other in a soft acid or base. Thus, according
to the HSAB principle, a correct matching of the hardness or its inverse,
softness, between an acid and a base gives rise to an extra stability of the
acid-base adduct. He further pointed out that in some cases, hard-soft is
considered to be a property of the acid or base; and in others, it is a prop-
erty of the acid-base interaction.