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TABLE 1.2
Classification of Lewis bases
Hard base or class (a) base
Borderline
Soft base or class (b)base
H 2 O ,
OH ,F ,CH 3 COO ,PO 4 3− ,SO 4 2− ,
Cl ,CO 3 2− ,ClO 4 ,NO 3 ,ROH,RO
,R 2 O,NH 3 ,RNH 2 ,N 2 H 4
C 6 H 5 (NH 2 ),
C 6 H 5 N,N 3 ,Br ,
NO 2 ,SO 3 2− ,N 2
R 2 S,RSH,RS ,I ,
SCN ,S 2 O 3 2− ,R 3 P, R 3 As, (RO) 3 P,
CN ,RNC,CO,C 2 H 4 ,C 6 H 6 ,H ,R
In the above classifi cation, a large number of Lewis acids were put
into one of two boxes, eventually labeled hard and soft by Pearson [9].
The same was done for a number of common bases. Due to the shortage
of data, all of one kind, a variety of criteria was used [9] such as bond
energies, equilibrium constants, rates of reaction, and the existence or non-
existence of certain compounds. Pearson opined that the HSAB classifi ca-
tion can be extended using the two groups of acids to group the bases into
hard and soft categories as well, using the principle that “hard acids tend
to bind hard bases; soft acids tend to bind soft bases.”
It should be noted that this statement is not an explanation or theory,
but it is a simple rule of thumb, which enables the chemists to predict
qualitatively the relative stability of acid-base adducts and such qualita-
tive description did not allow us for the quantifi cation of the hard and soft
behavior of acids and bases. Empirically, acids and bases were put into
one of two categories, but there was no way of rank ordering them within
the boxes. The main advantages of Pearson's classifi cation [9] are its wide
applicability and its ability to generalize certain chemical phenomena.
From polarizability consideration, Pearson had chosen to label class
(a) as hard and class (b) as soft. This parallelism was questioned by Wil-
liams and Hale [21]. They felt that it cannot be convincingly shown that
classes (a) and (b) are related to polarizability. Further, it is not clear that
such defi nitions as hard and soft really refer to the same physical proper-
ties in neutral acceptors as in ions. Such controversy is justifi ed since A
and B refers to the classifi cation of the metal ions in the aqueous medium
and Pearson's classifi cation applies in the gas phase.
Further, the various thermodynamic and bond energetic aspects of the
class (a) and class (b) types of behavior were critically examined in 1966
by Williams and Hale [21].
The hard-soft classifi cation of acid and bases has been popularized to
the scientist because, on the basis of hard-soft classifi cation, it is easier to
say harder or softer than more class (a) character or more class (b) char-
 
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