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do not follow the HSAB principle and pointed out some exceptions to the
HSAB principle.
Case 1: When one of the acids and one of the bases is very
strong: In such case the surpassing stability of the strong acid/
strong base (SA-SB) product can overwhelm the preference for
the hard acid-hard base (HA-HB) and the soft acid-soft base(SA-
SB) products. Here, the intrinsic strength of acids and bases is
measured using their electronic chemical potential. This exception
occurs only when one acid is much stronger than the other acid
and one base is much stronger than the other base.
Case 2: When one of the potential products of the double-ex-
change reaction is exceptionally stable: If the stability is due to
an orbital-specifi c interaction between one of the acids and one of
the bases, then the HSAB principle may be violated. For example,
a soft acid might be able to form a multiple bond to the hard base
but not to the soft base. This would preferentially stabilize the soft
acid-hard base product and an exception to the HSAB principle
might result.
Case 3: If the entropy of one or more of the potential prod-
ucts is very high: Then the maximum entropy products might be
favored over the minimum energy products. Ayers's [3] analysis
was based on the expectation that all the potential product mol-
ecules would have similar entropy. The fact that the HSAB prin-
ciple seems to work across the broad range of temperatures of
relevance to chemistry supports this assumption.
Case 4: If one or more of the potential products is preferen-
tially solvated : Then the favored product may be the molecule
that with the largest frees energy of solvation. From the famous
Le Chatelier's principle, we can say that if one or more of the
potential products is insoluble, then the reaction will proceed to
the direction of the insoluble product. Ayers [3] made an analysis
where he assumed that the free energy of solvation was similar
for all the potential product molecules. The fact that supports his
assumption is “the HSAB principle seems to work not only in the
gas phase, but across a broad range of solvents.”
Case 5: When the interaction between the reactive sites of the
acid and base is not driven by the dominant interaction be-
tween the acidic and basic reagents: For multidentate ligands
 
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