The document discusses hard and soft acid-base (HSAB) theory, which classifies acids and bases as either hard or soft based on their size, charge density, and polarizability. Hard acids and bases are small and nonpolarizable, preferring to interact with each other. Soft acids and bases are larger and more polarizable, preferring to interact with each other. This preferential hard-hard and soft-soft interaction leads to differences in properties like solubility, coordination preferences, and reaction kinetics. The theory has applications in understanding solubility trends, ligand substitution reactions, and the geochemical partitioning of elements.

1. Yashwantrao Chavan College of Science, Karad
Department of Chemistry
Dr. S. H. Burungale
Hard-Soft Acid-Base Theory

2. Hard-Soft Acid-Base Theory
1.1 Classification of acids and bases as hard and soft.
1.2 Theoretical bases of hardness and softness
1.3 Pearson’s HSAB concept.
1.4 Acid – Base strength and hardness and softness.
1.5 Application and limitations of HSAB principle.

3. Limitations of Lewis Acids and
Bases
failed to provide a definite and uniform
scale to measure the relative strength of
acid and base
In 1958 this was done by Ahrland , Chatt and
Davies by classifying the acids or bases into
general categories .Class – “a” and Class- “ b”
According to them the two categories of metal ions
(Lewis acids)are as follows –

4. Physical meanings behind HSAB
1 ionization energy & electronegativity
2 polarizability
3 HOMO-LUMO gap: large for hard
species, small for soft species
4 soft-soft interactions imply covalency;
and hard-hard interactions are for ionic
bonding

5. I) Class (a) – The metal ions which prefer to from
stable complexes with the ligands having donor
atom of the first members of Gr.15 th(N),16 th(O),
and 17 th(F) in the periodic table . .
Examples are – Alkali metals , Alkaline
earth metals and the first row transition
metals in high oxidation state (e.g.fe3+
,Co3+.Etc. belong to class (a) acids ) .

6. ii)Class (b )- The other metal ions which prefer
to from their most stable complexes with the
ligands having donor atom of the lower
members of Gr.15 th(P,As,Sb),16 th(S,Se,Te),
and 17 th(Cl,Br,I) in the periodic table . .
Lighter transition elements in low oxidation
state and heavier transition elements ,say Cu+ ,
Ag+,Hg+,Pt2+,Pd2+ etc. act as class (b) acids.

7. Ahrland , Chatt and Davies
Ahrland, Chatt, and Davies explained the class a and class
Class A Borderline Class B
H+, Li+ ,Na+, K+ Fe2+ Co2+ Ni2+ Cu+,Ag+,Au+,TI+
Be2+,Mg2+,
Ca2+
Cu2+ ,Zn2+,Pb2+ Hg+,Pd2+,Cd2+,
Cr2+,Cr3+,Al3+ SO2,BBr3 Pt2+,Hg2+,BH3,
Br2
SO3,BF3,BCl3 Br+, SO3,BF3,BCl3
HX(H-bonding) M0 (metal atoms) and
bulk metals
HX(H-bonding)

8. Class A
Acceptor atoms are marked by:
Class B
Acceptor atoms are marked by:
1) Small size 1) Large size
2) High positive oxidation state. 2) Zero or low positive oxidation
state
3) Absence of any outer electrons
Which are easily excited to
higher States.
3) Presence of several excitable
valence shell electrons.
4) Absence of d-electrons. 4) With nearly full d-electrons.
5) Usually light metal ions. 5) Mostly heavy metal ions.
6) Know as Lewis acids which
Are not easily polarizable Prefer
to coordinate with hard
6 6) Known as Lewis acids and
are easily polarizable. Prefer to
coordinate with soft bases.

9. Hard Borderline Soft
F-,OH-
,H2O,NH3
NO2,SO3,Br- H-,R-,CN-
,CO,I-,
CO32-,NO3-
,O2-,
N3-
,N2,C6H5N,
SCN-
,R3P,C6H6,
SO42-, PO43-
CIO4-,(Cl-)
SCN- R2S.
Lewis Base

10. Hard Soft
1) High electronegativity. 1) Low electronegativity
2) Low polarisability. 2) High Polarisability.
3) Presence of filled orbits
; empty orbitals may exist
at high energy level.
3) Partially filled orbitals,
empty orbitals are low-
lying.
4) These are anions or
neutral molecule known as
Lewis bases or ligands,
prefer to co-ordinate with
hard acids.
4) these are anions or neutral
molecules called similarly as
Lewis bases or ligands,
prefer to bind with soft
acids.

11. PEARSON’S CONCEPT
In 1963 R. G. Pearson extended and
generalized the qualitative correlation
between Lewis acids and Lewis bases
by classifying them into two
categories Hard and Soft.

12. The class –‘a’ metals which are small and less polarizable,
prefer to combine with non- metals or ligands which are also
small and not very polarizable , pearson called such metals as
Hard Acids and the corresponding ligands as hard Bases.
the class ‘b’metals having large size ,more or easily polarisable,
prefers to combine with non-metals or ligands having similar
properties Pearson called such metals as soft acids and the
ligands as soft base
The attempt of classification of acidsand bases as hard
and soft by Pearson is known as Hard and soft Acids and
Bases .(HSAB) or pearson’s concept

13. Hard
metals which are small and
less polarizable, prefer to
combine with non- metals
or ligands which are also
small and not very
polarizable , pearson called
such metals as Hard Acids
and the corresponding
ligands as Hard Bases.
Soft
’metals having large size
,more or easily
polarisable, prefers to
combine with non-metals
or ligands having similar
properties Pearson called
such metals as soft acids
and the ligands as soft
base
The attempt of classification of acidsand bases as hard
and soft by Pearson is known as Hard and soft Acids and
Bases .(HSAB) or pearson’s concept

14. Theorotical basis of hardness and softness
Several theories have been given to explain the
stability of complexes Formed by hard-hard and
soft-soft interactions. Some important theories are:
(a) Ionic and covalent bond theory
(b) - Bonding theory
(c) Pitzer’s theory

15. Ionic bond is formed
by the interaction of
hard acids and hard
bases
(a) Ionic and covalent bond theory
Covalent bond is formed by
the interaction of soft acids
and soft bases.
Electrostatic force of
attraction
Internulcear distance is
smaller
Highly stable
soft acids and soft bases have
larger size.
polarization effects
stronger covalent bond
high polarization

16. (b) -Π Bonding theory :
Mulliken (1955) and chatt (1956)
soft –soft interaction on the basis of π –bonding
Soft acids have low oxidation state and have a
large number of d- electrons.
strong tendency to form π –bonds
The polarization of soft acids and soft base also
favour π-bonding.

17. (c) Pitzer’s theory :
London dispersion energies stabilize a bond
between two large polarizable atom
These energies increase with an increase in the size
and polarizability .
soft-soft interactions are more stable as compared to
soft-hard interaction.

18. Hard and Soft Acids and Bases
The polarizability of an acid or base
plays a role in its reactivity. Hard acids and
bases are small, compact, and non-
polarizable.
Soft acids and bases are larger, with a
more diffuse distribution of electrons.

19. Hard and Soft Acids and Bases
In addition to their intrinsic strength,
Hard acids react preferentially with hard
bases, and soft acids react preferentially with
soft bases.

20. Aqueous Solubility
Silver Halides
Compound solubility product
AgF 205
AgCl 1.8 x 10-10
AgBr 5.2 x 10-13
AgI 8.3 x 10-17
AgX(s) + H2O(l) ↔ Ag+(aq) + X-(aq)

21. Solubility of Lithium Halides
LiBr> LiCl> LiI> LiF
LiF should have a higher ∆solv than the
other salts, yet it is the least soluble in water.
This is due to the strong hard acid (Li+)/hard
base (F-) interaction.

22. Thiocyanate Bonding
SCN- displays linkage isomerism as the
ligand coordinates to metals via the sulfur or
the nitrogen. Mercury (II) ion bonds to the
sulfur (a soft-soft interaction) whereas zinc ion
bonds to the nitrogen atom.

23. K for ligand exchange reactions
Compare:
[MeHg(H2O)]+ + HCl MeHgCl + H3O+
K= 1.8 x 1012
[MeHg(H2O)]+ + HF MeHgF + H3O+
K= 4.5 x 10-2

25. Hard and Soft Acids & Bases
There have been many attempts to
categorize various metal ions and anions to
predict reactivity, solubility, etc.
R.G. Pearson (1963) categorized acids and
bases as either hard or soft (using Kf values).
Hard acids bond in the order: F->Cl->Br->I-
Soft acids bond in the order: I- >Br- >Cl- > F-

26. Hard and Soft Acids & Bases
Hard acids or bases are compact, with the
electrons held fairly tightly by the nucleus.
They are not very polarizable. F- is a hard
base, and metal ions such as Li+, a hard acid.

27. Hard and Soft Acids & Bases
Large, highly polarizable ions are
categorized as “soft.” Iodide is a soft base,
and transition metals with low charge density,
such as Ag+, are considered to be soft acids.

28. Hard and Soft Acids & Bases
Hard acids tend to bind to hard bases.
Soft acids tend to bind to soft bases.

29. • Predict the solubility (high or low) of silver
fluoride, silver iodide, lithium fluoride and
lithium iodide using the hard-soft acid/base
approach. Identify each Lewis acid and Lewis
base, and categorize each as hard or soft.

30. Charge Density – Hard Acids
Hard acids typically have a high charge
density. They are often metal ions with a
(higher) positive charge and small ionic size.
Their d orbitals are often unavailable to
engage in π bonding.

31. Charge Density – Soft Acids
Soft acids typically have lower charge
Their d orbitals are available for π bonding.
Soft acids are often 2nd and 3rd row transition
metals with a +1 or +2 charge, and filled or
nearly filled d orbitals.

32. Acids
Hard Acids Borderline Soft Acids
H+, Li+, Na+, K+
Be2+, Mg2+, Ca2+
BF3, BCl3, B(OR)3 BBr3,B(CH3)3 BH3,Tl+, Tl(CH3)3
Al3+,Al(CH3)3,AlCl3,AlH3
Cr3+,Mn2+, Fe3+, Co3+ Fe2+,Co2+,Ni2+ Cu+,Ag+, Au+,
Cu2+,Zn2+,Rh3+ Cd2+,Hg2
2+,
Ir3+, Ru3+, Os2+ Hg2+, Pd2+,Pt2+,
SO3 SO2 Pt4+

33. Acids – Effect of Oxidation state
Hard Acids Borderline Soft Acids
H+, Li+, Na+, K+
Be2+, Mg2+, Ca2+
BF3, BCl3, B(OR)3 BBr3,B(CH3)3 BH3,Tl+, Tl(CH3)3
Al3+,Al(CH3)3,AlCl3,AlH3
Cr3+,Mn2+, Fe3+, Co3+ Fe2+,Co2+,Ni2+ Cu+,Ag+, Au+,
Cu2+,Zn2+,Rh3+ Cd2+,Hg2
2+,
Ir3+, Ru3+, Os2+ Hg2+, Pd2+,Pt2+,
SO3 SO2 Pt4+

34. Bases
Hard Bases Borderline Soft Bases
F-, Cl- Br- H-, I-
H2O, OH-,O2- H2S, HS-, S2-
ROH, RO-, R2O, CH3CO2
- RSH, RS-, R2S
NO3
-, ClO4
- NO2
-, N3
- , N2 SCN-, CN-,RNC, CO
CO3
2-,SO4
2-, PO4
3- SO3
2- S2O3
2-
NH3, RNH2 C6H5NH2, pyr R3P, C6H6

35. Bases – effect of Oxidation State
Hard Bases Borderline Soft Bases
F-, Cl- Br- H-, I-
H2O, OH-,O2- H2S, HS-, S2-
ROH, RO-, R2O, CH3CO2
- RSH, RS-, R2S
NO3
-, ClO4
- NO2
-, N3
- , N2 SCN-, CN-,RNC, CO
CO3
2-,SO4
2-, PO4
3- SO3
2- S2O3
2-
NH3, RNH2 C6H5NH2, pyr R3P, C6H6

36. Effect of Linkage Site
SCN- vs. NCS-
The nitrogen tends to coordinate with
harder acids such as Si, whereas the sulfur
tends to coordinate with softer acids such as
Pt2+.

37. Effect of Oxidation Number
Cu2+/Cu+ on acid hardness
SO3/SO2 on acid hardness
NO3
-/NO2
- on base hardness
SO4
2-/SO3
2- on base hardness

38. Acid or Base Strength
It is important to realize that hard/soft
considerations have nothing to do with acid or
base strength. An acid or a base may be hard
or soft and also be either weak or strong.
In a competition reaction between two
bases for the same acid, you must consider
both the relative strength of the bases, and
the hard/soft nature of each base and the
acid.

39. Acid or Base Strength
Consider the reaction between ZnO and
LiC4H9.
ZnO + 2 LiC4H9↔ Zn(C4H9)2 + Li2O
Zinc ion is a strong Lewis acid, and oxide
ion is a strong Lewis base.

40. Acid or Base Strength
Consider the reaction between ZnO and
LiC4H9.
ZnO + 2 LiC4H9↔ Zn(C4H9)2 + Li2O
Zinc ion is a strong Lewis acid, and oxide
ion is a strong Lewis base. However, the
reaction proceeds to the right (K>1), because
hard/soft considerations override acid-base
strength considerations.
soft -hard hard -soft soft -soft hard -hard

41. The Nature of the Adduct
Hard acid/hard base adducts tend to have
more ionic character in their bonding. These
are generally more favored energetically.
Soft acid/soft base adducts are more
covalent in nature.

42. • As the adduct forms, there is usually a change
in geometry around the Lewis acid site.
BX3 + N(CH3)3  X3B-NMe3
The stability of the adduct is:
BBr3 > BCl3> BF3
This order seems opposite of what would
be expected based on halogen size or
electronegativity.

43. Other Considerations
empty 2p orbital
filled orbitals
The reactivity pattern
suggests some degree of
π bonding in BF3.

44. Other Considerations
• Steric factors can play a role. An example is
the unfavorable reaction between :N(C6H5)3
and BCl3. The large phenyl groups interact
with the chlorine atoms on boron to
destabilize the product.

45. Applications of Hard/Soft Theory
The Qual Scheme, a series of chemical
reactions used to separate and identify the
presence of dozens of metal ions, is based
largely on the hard and soft properties of the
metal ions.
The softer metals are precipitated out as
chlorides or sulfides, with the harder ions
formed as carbonates.

47. Evidence in Nature
In geochemistry, the elements in the
earth’s crust are classified as lithophiles or
chalcophiles.
The lithophile elements are typically found
as silicates (bonded via the O atom): Li+, Mg2+,
Ti3+, Al3+ and Cr2+,3+. These are hard Lewis
acids.

48. Evidence in Nature
The chalcophile elements are typically
found as sulfides or bonded to Se2- or Te2-.
They include: Cd2+, Pb2+, Sb3+, and Bi3+. These
are soft Lewis acids. Zinc ion, which is
borderline, is typically found as a sulfide.