Page 176 - Mechanism and Theory in Organic Chemistry

Lewis Acids and Bases 165
more strongly bonded to A than it is to B. In this book we shall maintain the

distinction between the two definitions and shall follow the usual convention that
the unmodified term acid will refer to a Bransted acid, while Lewis acids will be
specified as such.
Strengths of Lewis Acids and Bases
Because the Lewis acid-base concept is an extremely useful one in chemistry,
quantitative relationships of the types discussed in the previous sections for
Bransted acids and bases would be helpful. The task of classifying the Lewis acids
and bases according to some criterion of strength has nevertheless proved to be a
difficult one, and methods being developed still yield largely qualitative results.
Brsnsted acid-base reactions always involve transfer of a proton; this common
feature allows meaningful quantitative comparisons of strengths to be made.
Different Lewis acid-base reactions, on the other hand, do not necessarily have
any feature in common, and the result is that the term "strength" does not have a
well-defined meaning.
The problem may be illustrated by a simple example.l1° Suppose that we
wish to compare the "coordinating power " of two Lewis bases, say an amine,
NR,, and a phosphine, PR,. We might do this by comparing the equilibrium

K '
A + B' AB' (3.63)
constants for Reactions 3.62 and 3.63 of the two bases, B and B', with the same
Lewis acid, for example BF,. Quantitative data are not always available; but it is
often possible to make qualitative decisions about orders of reactivity. The
information we have shows that the nitrogen base should be judged to have the
greater coordinating power, since the equilibrium constant is greater for the
formation of nitrogen base complex.111 A similar qualitative result is found with
H+ as the reference acid.l12 If, on the other hand, the equilibrium constants for a
nitrogen and a phosphorus base with Ag+ are measured, it is found that with
respect to this Lewis acid the phosphine has much greater coordinating power
than does the amine. A similar situation arises with another set of bases, the
halide ions. If H + is taken as the reference acid, fluoride is the most effective base
in solution, followed by chloride, bromide, and iodide. With silver ion, however,
the order is exactly reversed; iodide forms the most stable complex and fluoride
the least stable.l13

Hard and Soft Lewis Acids and Bases
Despite the apparent chaos of the picture presented by these results, it is possible
to find some qualitative relationships that are useful. Schwarzenbach,l14 and also

110 S. Ahrland, J. Chatt, and N. R. Davies, Quart. Reu. (London), 12, 265 (1958).
111 W. A. G. Graham and F. G. A. Stone, J. Znorg. Nucelar Chem., 3, 164 (1956).
112 See Table 3.13 for solution data and Table 3.18 for gas-phase data.
113 See note 110.
114 (a) G. Schwarzenbach, Experientia, Suppl., 5, 162 (1956); (b) G. Schwarzenbach, Aduan. Znorg.