8/12/2019 HSAB_3
1/1
1827
Orgunic and Biologicul Chemistry
Application of the Principle of Hard and Soft Acids and Bases
toOrganic Chemistry
Ralph G .Pearson and Jon Songstad
Contribution fr om the Department o Chemistry, Northwestern
University,Euanston, Illinois 60201. Received November2, 1966
Abstract: The principle of hard and soft acid and basesHSAB
principle) is applied to organic chemistry.,Organicmolecules are
viewed as Lewis acid-base complexes and their relative thermodynam
ic stability explainedin terms of two facto rs. One is the tendency
of intrinsically strong acids to coordinate to the strongest bases.
Thesecond is the special stabilization of combinations of hard
acids and bases,or so t acids and bases. The symbioticprinciple is
illustrated, which states that the re is an extra stabilizationif
several soft bases (ligands)or several hardbases cluster abou t a
single acidic atom. The same two principles are applied to rates of
nucleophilic and elec-trophilic substitu tion eactions in organic
chemistry.
ecently2a generalization was proposed w hich makes
R i t possible to correlate a great many phenomenain various
areas of chemistry. Use is made of the con-cept of generalized, or
Lewis, acids and bases. Th egeneralization may be called the
principle of hardand soft acids and bases (HSAB). It states that
hardacids prefer to coordinateto hard bases and soft acidsprefer to
coordinateto soft bases.
These terms are qualitatively defined in the followingways: soft
base-donor at omis of high polarizability,low electronegativity,
easily oxidized, and associatedwith empty, low-lying orbitals hard
base-donor atomis of low polarizability, high electronegativity,
hard tooxidize, and associated with empty orbitals of highenergy
and hence inaccessible ; soft acid-the acce ptoratom is of low
positive charge, large size, and hasseveral easily excited ou ter
electro ns; har d acid-ac-ceptor atom is of high positive charge,
small size, anddoes not have easily excited o uter electron s
Operationa lly, acids may be defined by following thep ro ce du
re s of S c h ~ a r z e n b a c h ~nd Ahrland, Chatt, andDavies.4
These workers divided metal ions (which areLewis acids) into tw o
classes calledA and B by Schwar-zenbach and a and b by Ahrland,
Chatt, and Davies.Ha rd acids follow the same p attern as class a
metal ions,and soft acids show the pattern of class b metal
ions.For complexes with different don or atom s, the
followingsequences of stabilities are found.
N >>P >As > Sbhard { >>S > Se > TeF >
CI > Br > I
N As > Sbsoft { < S - e Te
F < CI < Br I
Soft bases might be operationally defined by con-sidering th e
equilibrium6
1) Chemistry Departm ent, Bergen University, Norw ay. Supp
ortedby the Royal N orwegian C ouncil for Scientific and Industrial
Research.
2) R . G. Pearson, J . Am. Chem. SOC.,5 , 3533 1963); Science,
151,172 1966).
3 ) G. Schwarzenbach, Experientia Suppl., 5, 162 1956);
Aduan.
( 4 ) S Ahrland, J. Chatt , and N. R . Davies, Quart. Rev.
(London),12,Inorg. Chem. Radiochem., 3, 251 1961).
265 1958).
CHSHg+(aq) BH+(aq) CHaHgB+(aq) H+(aq) 1)If the equilibrium
constantfor this reaction is muchgreater than unity, the base B is
soft. If it is near unity,or less tha n unity, the baseis hard. The
proton is thesimplest hard acid and the methylmercury cation is
oneof the simplest soft acids. TableI contains a listing ofhard and
s oft bases for later reference.
If the equilibrium constants of eq1 are used t o ranka series of
bases, the following order of decreasingsoftness is o btained.I-
> Br- > CI- > S2 > RS > CN- > HzO > NH3
-- > OH-
It turns out that this is not a universal order since achange in
one of the reference acids will give a dif-
ferent series. The reason for this may be seen by con-sidering
the generalized acid-base exchan ge reac tion.
A:B A :B A:B A :B 2)
We expect such a reaction to proceed such that thestrongest
acid, A, is found coordinated to the strongestbase, B. The terms
hard and soft do not mean thesame as strong and weak. Thu s an acid
is charac-terized by at least two properties, its strength and
itshardness, or softness; the same is true fora base.It is well
known that there isn o universal order of acidor base streng th;
still we recognize th at som e Lewisacids, such as Hf, are much
stronger than other acids,such as 12, or thatH- is a much stronger
base thanHzO.The HSAB principle then states that there is an
extrastabilization in A :B if both the acid an d base are hard,or
if b oth are soft.6
We can usually recognize hardness or softness in aqualitative
way by examining an acidor base, par-ticularly the donor or
acceptor atom s. The situationmay be som ething like that for the
terms solvent polar-
5) G Schwarzenbach and M. Schellenberg,Helu. Chim Acta, 48,28
1965); G Schwarzenbach,Chem. Eng. News, 43 , 92 ( M a y 31,
1965).
(6) Thus the equilibrium constant for the reactionA :B A :
Bmight be characterized by an equatio n such as logK = S A S B
UAUBThe factors S nd SB re strength factors for the acid and base;U
n dUB are softness factors. For a hard acid or base, wouldbe
negative;for a soft acidor base, would be positive; see R. S. Drago
and B. B.Wayland, J . Am. Chem.Soc . 87, 3571 1965).
Pearson, Songstad ] Hard and Soft cids and Bases
