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bonding. It is then applied and offered as an explanation for observations
on systems in which kinetic control, entropy of adduct formation, and en-
thalpic and entropic effect such as solvation effects, ion-pairing effects,
and/or lattice energy effects are large and could even dominate the obser-
vation. Drago [38] further commented “when HSAB considerations are
employed on these systems, one is implying that the soft-soft or hard-hard
interactions, which supposedly infl uence the bonding, dominate the chem-
istry.”
According to Drago [38], the double-scale approach (
E
and
C
ap-
proach
)
predicts the strength of interaction quantitatively on systems in
which the data used are related to the strength of binding. If the HSAB
ideas are correct models for the strength of bonding, it must be capable
of being expressed in a mathematical form and do well on data related
to the strength of interaction. If it does not, it should be discarded and
replaced by a more correct interpretation of the strength of binding. Drago
[38] again stated that knowledge of the factors infl uencing the strength of
interaction is important in correlating trends in reactivity and in suggest-
ing new syntheses or experiments. This has been amply demonstrated by
the wide acceptance and utilization of the HSAB ideas. The more correct
model should be even more powerful in this kind of application.
Pearson [37] took issue with Drago's quantitative statement of the rule
that “hard acids prefer to bind to hard bases and soft acids prefer to bind to
soft bases” with the equation:
-ΔH =
H
A
H
B
+ (
K
−
H
A
)(
K
/
H
B
)
(13)
This rather obvious mathematical translation of the above rule was crit-
icized because Drago et al. [34-36] failed to incorporate intrinsic strength.
Again, Pearson [37] stated that the matter has never been clearly de-
fi ned; it is diffi cult to see how his quantitative statement of HSAB can
possibly work. The terms
-ΔH
and log K can be interchangeably used in
HSAB; he proposed to write Eq. (10) as follows:
-ΔH
=
S
A
S
B
+ -σ
A
σ
B
(14)
An equation of this form must fit the enthalpy data as well as the
E
and
C
equation:
-ΔH
=
C
A
C
B
+ -
E
A
E
B
(15)
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