Page 177 - Mechanism and Theory in Organic Chemistry

Ahrland, Chatt, and Davies,l15 classified Lewis acids into two categories, &s a
and Class b. Class a acceptors ar- their most -- stable complexes
-
with donors ofthe ktrbw&it~i pe~adic~~bk N, 0, and F. ~l%Zi~ii%s-
cmlex best with d n n n r ~ , ClJ3qL116 ~ ,
~
This classification scheme for Lewis acids has been generalized and ex-
tended by R. G. Pearson.l17 He proposes that each Lewis acid and base be
characterized by two parameters, one of which is referred to as strength and the
other of which is called softness. Thus the equilibrium constant for a simple acid-
l
base reaction (Equation 3.62) would be a function of four parameters, two for
each partner. 1
The next step in Pearson's argument is to classify acids and bases as hard or
soft according to their properties. Hard acids correspond roughly in their behavior
to the Class a acids of Schwarzenbach and of Ahrland, Chatt and Davies. They
are characterized by small acceptor atoms that have outer electrons not easily
excited and that bear considerable positive charge. Soft acids have acceptor atoms !
of lower positive charge, large size, and with easily excited outer electrons. Hard i
and soft bases are defined analogously. Hard bases contain highly electronegative I
donor atoms of low polarizability,ll8 are typically difficult to oGidize, and have
no empty low-energy orbitals available; soft bases are polarizable, have less
electronegative donor atoms, and have empty orbitals of low energy and electrons
that are more easily removed by oxidizing agents. Table 3.20 gives Rearson's
classification of acids and bases into the hard and soft categories.
\
Having defined the terminology, we may now state Pearson's principle o
hard and soft acids and bases (commonly abbreviated HSAB principle) : Hard acids
prefer to bind to hard bases and soft acids prefer to bind to soft bases.llg

of the HSAB principle, there are difficulties. The proposed scheme is one in I
Shortcomings of the HSAB principle Despite the apparent success
which two parameters, strength and softness, characterize each acid and each
base. Although published discussions120 have been specific about how softness is
determined, they have said much less about the strength parameter, and most of
the applications and examples have been considered mainly from the point of
view of the hardness or softness of the acids and bases concerned.
The only satisfactory way to handle the situation would appear to be to
establish numerical scales for both strength and hardness. Although limited work
along these lines has been done,lZ1 it does not appear possible to extend the
quantitative correlations to cover the wide range of reactions that seem to fit in at

Chem. Radiochem., 3, 257 (1961); (c) G. Schwarzenbach and M. Schellenberg, Helv. Chim. Acta, 48,
28 (1965).
115 See note 110, p. 165.
116 See also J. 0. Edwards and R. G. Pearson, J. Amer. Chem. Soc., 84, 16 (1962).
117 (a) R. C. Pearson, J. Amer. Chem. Soc., 85, 3533 (1963); (b) R. G. Pearson and J. Songstad, J.
Amer. Chem. Soc., 89, 1827 (1967); (c) R. G. Pearson, Survey of Progress in Chemistry, 5, 1 (1969).
118 See Section 2.4.
118 See note 117 (c).
lz0 See note 117.
lz1 See, for example: (a) J. 0. Edwards, J. Amer. Chem. Soc., 76, 1540 (1954); (b) R. S. Drago and
B. B. Wayland, J. Amer. Chem. Soc., 87, 3571 (1965).