Bases Duras Y Blandas

JOURNAL
O F T H E AMERICAN CHEMICAL SOCIETY
Registered i n U . S. P atent Ofice

@ C opyritht, 1 963, b y the American Chemical Society

VOLUME 5, N UMBER2
8
2

N OVEMBER 1963
20,

P HYSICAL AND INORGANIC CHEMISTRY
[CONTRIBUTION
FROM

T HE

D EPARTMENT CHEMISTRY,
OF
NORTHWESTERN
UNIVERSITY,
EVANSTON,
ILL.]

Hard and Soft Acids and Bases
B Y RALPH . PEARSON
G
R ECEIVEDJ UNE 14, 1963

A n umber of Lewis acids of diverse types are classified as (a) or ( b ) following the criterion of Ahrland, Chatt,
and Davies. O ther, auxiliary criteria are proposed.
Class ( a ) acids prefer to bind to “hard” or nonpolarizable
bases. Class ( b ) acids prefer to bind t o “ soft” or polarizable bases. Since class (a) acids are themselves “hard”
and since class ( b ) acidsare “soft” a simple, useful rule is proposed: hard acids bind strongly to hard bases
and soft acids bind strongly to soft bases. The explanations for such behavior include: ( 1) various degrees
of ionic and covalent u-binding; (2)a -bonding; (3) electron correlation phenomena; (4) solvation effects.

I n a recent publication’ the rate data for the generalized nucleophilic displacement reactionwere reviewed
and analyzed.

s + S-x +s -s

+x

( 1)

Here N is a nucleophilic reagent (ligand, Lewis base)
and S-X is a substrate containing a replaceable group X
(also a base) and an electrophilic atom (Lewis acid) S.
O ther groups may also be bound to S. I t was found
that rates for certain substrates, S-X, were influenced
chiefly by the basicity (toward the proton) of N,
andother substrates had rates which depended chiefly
on the polarizability (reducing power, degree of unsaturation) of N .
I n this paper the equilibrium constants of eq. 1
will be considered, instead of the rates.
9 ( base)

+ S-X (acid-base d- K-S
complex)

(acid-base
complex)

+ X ( base)
(2)

T hus the relative strengths of a series of bases, N ,
will be compared for various acids,S. T he reference base
X will be constant for each comparison. In solution X
m ay simply be the solvent, and in the gas phase X m ay
be completely absent. Thus the discussion of equilibrium constants is concerned only with the stability of
acid-base adduct N-S a nd the stability of the free (or
solvated) base N. The nature of N-S m ay be that of a
stable organic or inorganic molecule, acomplex ion, or a
charge transfer complex. In all cases it will be assumed that N is acting in part as an electron donor and
S a s an electron acceptor so t hat a coordinate, covalent
bond between N and S is formed. Other types of interaction, sometimes stronger, sometimes weaker, may
occur. T hese will be discussed later.
I n t erms of equilibria, rather than rates, i t again turns
out thatvarious substrate acids fall into two categories: those that bind strongly to bases which bind
strongly to the proton, that is, basic in the usual
sense ; those that bind strongly to highly polarizable
or unsaturated bases, which often have negligible
proton basicity. Division into these two categories is
not absolute and intermediate cases occur, but the
classification is reasonably sharp andappears to be
quite useful. I t will be convenient to divide bases into
(1) J 0 E dwards and R G P carson, J A m Chem S o c , 8 4, 16 ( 1962)

t wo categories, those that are polarizable, or “soft,”
and those that are nonpolarizable, or “hard.”2 Now it
is possible for a base to be both soft and strongly
binding toward the proton, for example, sulfide ion.
Still it will be true thathardness is associated with good
proton binding. For example, for the bases in which
the coordinating atom is from groups V , V I, a nd V I1
( the great majority of all bases), the atoms F, 0 , a nd N
are the hardest in each group and also most basic to
the proton. The reason for this has been discussed in
reference 1. T he atoms in each group become progressively softer with increasing atomic...