Pearson's Hard Soft Lewis Acid Base (HSAB) principle classifies Lewis acids and bases as hard, soft or borderline based on their ability to form ionic or covalent complexes. According to the principle, hard acids prefer hard bases and form ionic complexes, while soft acids prefer soft bases and form covalent complexes. However, the HSAB model oversimplifies reactions and introduces ambiguities, as no single physical parameter determines hardness. A more nuanced analysis is needed to understand complex Lewis acid-base interactions.

1. Pearson's Hard Soft [Lewis] Acid Base
The Hard Soft [Lewis] Acid Base Principle
In the nineteen sixties, Ralph Pearson attempted to explain the differential affinity of electron
pair donating Lewis bases towards electron pair accepting Lewis acids, ie. Lewis acid/base
complexation:
A + B –> A-B
by classifying Lewis acids and Lewis bases as hard, borderline or soft.
According to Pearson's hard soft [Lewis] acid base (HSAB) principle:
Hard [Lewis] acids prefer to bind to Hard [Lewis] bases
and
Soft [Lewis] acids prefer to bind to Soft [Lewis] bases
Pearson's HSAB Species
Pearson's Hard Lewis Acids:
Pearson's Borderline Lewis Acids:
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2. Pearson's Hard Soft [Lewis] Acid Base
Pearson's Soft Lewis Acids:
Pearson's Hard Lewis Bases:
Pearson's Borderline Lewis Bases:
Pearson's Soft Lewis Bases:
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3. Pearson's Hard Soft [Lewis] Acid Base
In 1968, G. Klopman quantified Pearson's HSAB principle using frontier molecular orbital
(FMO) theory. (The Klopman equation is discussed, here. ) Klopman proposed that:
Hard [Lewis] Acids bind to Hard [Lewis] Bases to give charge-controlled
ionic complexes
and
Soft [Lewis] Acids bind to Soft [Lewis] Bases to give FMO-controlled
covalent complexes
Combining Pearson's and Klopman's ideas:
• Hard Lewis acids:
Atomic centres of small ionic radius and with net positive charge.
Species do not contain electron pairs in their valence shells, they
have a low electron affinity and are likely to be strongly solvated.
High energy LUMO.
• Soft Lewis acids:
Large radius, low or partial (delta+) positive charge.
Electron pairs in their valence shells. Easy to polarise and oxidise.
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4. Pearson's Hard Soft [Lewis] Acid Base
Low energy LUMOs but large magnitude LUMO coefficients.
• Hard Lewis bases:
Small, highly solvated, electronegative atoms: 3.0-4.0.
Species are weakly polarisable and are difficult to oxidise. Low
Low energy HOMO.
• Soft Lewis bases:
Large atoms of intermediate electronegativity: 2.5-3.0.
Easy to polarise and oxidise.
High energy HOMOs but large magnitude HOMO coefficients.
• Borderline species have intermediate properties.
• It is not necessary for species to possess all properties.
The Pearson approach is very successful when comparing pairs of species:
Sodium ion Na+ is harder than the silver ion Ag+
Alkoxide ions, RO–, are harder than thioanions, RS–
Copper(II) ion, Cu2+, is harder than copper(I) ion, Cu+
The nitrogen anion end of the ambidentate cyanide ion, CN–, is harder than the
carbon anion end, NC–
This type of analysis can be very useful. For example, beta-propiolactone is ring opened by
nucleophilic Lewis bases, however, attack can occur at two positions:
• Harder nucleophiles like alkoxide ion, R-O–, attack the acyl (carbonyl) carbon.
• Softer nucleophiles like thioanion, R-S–, attack the 3-position carbon.
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5. Pearson's Hard Soft [Lewis] Acid Base
This graphic is captured from The Chemical Thesaurus, here. There are a
number of examples of this type in this reaction chemistry database.
Problems...
However, there is a problem.
While the Pearson-Klopman HSAB model is not "wrong"... it does grossly oversimplify the
reaction chemistry. The reason is that no physical parameter correlates with hardness over
Pearson's chosen set of species.
And as a result, the model introduces ambiguities. How similar are:
Pearson's hard Lewis acids: [NH4]+ H+ CO2 Cs+ Cu2+
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6. Pearson's Hard Soft [Lewis] Acid Base
Pearson's soft Lewis bases: H– R2S: R– benzene
Even more importantly, the interesting hard-soft interactions and complexations are simply not
discussed by Pearson's model. What about:
Sodium hydride
Lithium aluminium hydride
Lead(IV) acetate
Methyl lithium
Borane-THF
Triethyloxonium tetrafluoroborate
Ferrocene
The one-dimensional hard-borderline-soft continuum or Pearson's analysis actually has
the effect of blurring much of the rich, linear (predictable) behaviour which can be
found in Lewis acid/base reaction chemistry space.
The chemogenesis analysis, here, avoids and explains the pitfalls of the
Pearson approach.
The HSAB Papers
R.G.Pearson, J.Am.Chem.Soc., 85, 3533-3543, 1963
R.G.Pearson, Science, 151, 172-177, 1966
R.G.Pearson, Chem. Br., 3, 103-107, 1967
R.G.Pearson, J.Chem.Ed., 45, 581-587, 1968
G.Klopman and R.F.Hudson, Theoret. Chim. Acta, 8, 165, 1967
G.Klopman, J.Am.Chem.Soc., 90, 223-234, 1968
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7. Pearson's Hard Soft [Lewis] Acid Base
Also look here.
© Mark R. Leach 2003
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