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Using the condition μ A = μ B and the Eq. (37), it can be shown that
I A + A A = I B + A B
(42)
and
The ionization potential effect: I B - I A = η B - η A
(43)
where A A and A B are the electron affi nity of the A and B , respectively.
Thus, if both η B and η A are small, and/or if they are equal, then A and
B have the same ionization potential in the molecule AB, that is, equaliza-
tion of ionization potential occurs. In orbital language, this means that the
one-electron energies of atomic orbitals on A and B have been made equal,
or nearly equal. In both molecular orbital theory and valence bond theory,
if orbital sizes are not too disparate, equal orbital energies always favor
strong covalent bonding [46].
Parr and Pearson [46] further added that
(i) Soft-soft interactions are largely covalent: In soft acid-soft base
adduct formation process, the two bonding electrons have compa-
rable probabilities of being on A or B. Again this combination is
favored by the ionization potential effect ( I B I A = η B − η A ). These
two combined effects show that the covalent bonding is dominant
in soft-soft combinations and that good bonding will result.
(ii) Hard-hard interactions are largely ionic: In hard acid-hard base
combinations, since there is little electron transfer from B to A,
the bonding electrons must, on the average, remain on B resulting
ionic bond formation. The chemical potential equalization is still
favored that if both η B and η A are large, they might cancel each
other. But this does not necessarily produce strong covalent bond-
ing. The characteristics of a hard acid are high positive charge and
small size of the cation which favor electrostatic interaction with
B, which retains most of its negative charge, and which presents a
favorable dipole interaction with a lone pair of electrons pointing
at A.
(iii) Hard-soft combinations are unfavorable: For a hard-soft com-
bination, on the contrary, stability is enhanced neither by the charge
transfer nor by the condition favoring covalent bonding.
 
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