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Although, Ayers [3] has always been doubtful of the idea that “match-
ing” the local softnesses of the acidic and basic reactive sites favors re-
activity and pointed out some exceptions of the “matching” concept. “In
spite of the value of the local softness at the site, it seems that hard acids
usually prefer the most negatively charged reactive site on a base. Even
when the two reactive sites have similar negative charges, a hard acid may
prefer the site with the larger local softness [3].”
1.2.20.1
ELECTROSTATIC AND ELECTRON-TRANSFER EFFECTS
Ayers [3] explained the contribution of the physical properties of hard
reagents to the chemical reaction energy between acids and bases, and he
also observed that this consideration supports the HSAB principle. It was
stated that the variation principle for the electron density also plays a role
in the HSAB principle:
when an acid and a base react, electron density moves from the base to
the acid until the energy of the product is minimized.
In the case of such reaction, the electron-transfer process lowers the
energy of the system (ΔE
el
) by
ΔE
el
= -(μ
B
− μ
A
)
2
/2(η
A
+ η
B
) = −(χ
B
− χ
A
)
2
/2(η
A
+ η
B
)
(84)
Here μ, χ, and η denote the chemical potential, electronegativity, and
the hardness of the acid (A) and the base (B) involved in the reaction,
respectively.
The above Eq. (84) computes the contribution of electron transfer to
the reaction energy between an acid and a base.
The preference for the hard acid/hard base and soft acid/soft base prod-
ucts in the double-exchange reaction, Eq. (77), can be easily explained
by the electron-transfer effects if there were no other contributions to the
reaction energy.
It was shown [69] that for the single-exchange reactions with a soft
reagent, Eqs. (78) and (80), the electron-transfer contribution to the en-
ergy favors the soft acid—soft base product over the “mixed” hard acid/
soft base or soft acid/hard base alternative. This is a support to the HSAB
principle.
However, there is some contradiction to the HSAB principle. The
electron-transfer contribution to the reaction energy also predicts that the
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