Chemistry Reference
In-Depth Information
After the formation of the molecule AB, the chemical potentials of A and B are
equal in the molecule (electronegativity/chemical potential equalization principle).
Thus
μ
A
=
μ
A
o
+ 2η
A
∆N = μ
B
=
μ
B
o
- 2η
B
∆N
(88)
The shift of charge/electron transfer,
∆N = (μ
B
o
-
μ
A
o
)/2 (η
A
+ η
B
)
(89)
as,
χ
= -
μ
Thus,
∆N = (χ
0
A
-
χ
0
B
)/2(η
A
+ η
B
)
(90)
The corresponding energy change was calculated as follows:
∆E = (E
A
-E
0
A
) + (E
B
-E
0
B
)
= - (1/2)(
μ
B
o
-
μ
A
o
)
∆N
(91)
or,
∆E = - (χ
0
A
-
χ
0
B
)
2
/4(η
A
+ η
B
)
(92)
As the acid must be more electronegative than base,
(χ
0
A
-
χ
0
B
)
is always positive,
an energy lowering results from the electron transfer process. The difference in abso-
lute electronegativity drives the electron transfer, and the sum of the hardness param-
eter acts as a drag or resistance. In other words, the differences in electronegativity
drive the electron transfer and the sum of the absolute hardness parameters inhibits
electron transfer.
If both acid and base are soft, (η
A
+ η
B
) is a small number, and for a reasonable
difference in electronegativities, ∆E is substantial and stabilizing. This explains the
HSAB principle, meanwhile, it seems safe to say that it explains a part: soft prefers
soft. But if both acid and base are hard, there is little electron transfer and energy
stabilization from electron transfer, for a given difference in electronegativities. Parr
and Pearson [69] commented—“
This result seems paradoxical
” and there is the need
of the second effect--the formation of the chemical bond. Parr and Pearson [69] also
commented that the consideration “
soft-soft interactions are largely covalent, and that
hard-hard interactions are largely ionic
” is not always novel. Providing η
A
and η
B
are
both small, the stabilization of A: B adduct is explained [70] by the concept of double
bonding. The concept of double bonding resembles with the π-bonding theory of Ah-
rland, Chatt, and Davies [71-73] who used it for explaining various metal ion-ligand
preferences. In case of the adduct formation between a hard acid and a hard base nor-
mally “little two-way electron transfer” occur. It should be notated that Pearson also
showed that there will be little one-way transfer from B to A, if η
A
and η
B
are large. For
cationic acids the probability of double bonding is greatly reduced. The main source of
bonding 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
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