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Parr and Pearson [46] also opined that the consideration that the soft-
soft interactions are largely covalent, and that hard-hard interactions are
largely ionic is not always novel.
1.2.13 THE CONCEPT OF DOUBLE BONDING
Pearson [54] explained the adduct formation between a neutral acid and
a neutral base. Provided that η
A
and η
B
are both small, the stabilization of
A:B adduct can be explained by double bonding. The concept of double
bonding resembles the π-bonding theory of Chatt and his coworkers [19]
who used the concept of π-bonding for explaining various metal ion-li-
gand preferences. During the adduct formation between a hard acid and a
hard base, normally little two-way electron transfer occurs. If η
A
and η
B
are
large, Pearson [9] also showed that there will be a one-way transfer from
B to A. The probability of double bonding is greatly reduced for cationic
acids. The main source of bonding that involves cationic acids will come
from ionic bonding or ion-dipole bonding. Neutral molecules are the most
likely to have two-way electron transfer. The unbiased values of (χ
0
A
- χ
0
B
)
for the neutral molecule determine the direction of net electron transfer.
The total amount of electron transfer is governed by the sum of hardnesses
of the acid and the base, (η
A
+ η
B
), and a small value of the summation is
favorable for maximum covalent bonding.
Pearson [54] further commented that Eq. (39) measures the initial ef-
fects of bringing A and B into interaction with each other, and Eq. (39) is
not that successful/suitable for calculating the net charges on the acid A
and the base B.
Considering the above effects and using semiempirical molecular or-
bital theory, including ionic bonding and covalency. Pearson [54] derived
an equation for ΔN as follows:
ΔN = (χ
A
~ χ
B
)/[2(η
A
+ η
B
) − (1/
R
) − {2β/(
N
A
N
B
)
½
}]
(44)
Where β
is the one-electron exchange integral, and hence is always nega-
tive;
R
is the inter nuclear distance between
A
and
B
; and
N
A
and
N
B
are the
final numbers of electrons on
A
and
B
atoms, respectively.
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