Hard and Soft Acids and Bases (HSAB) Theory is a qualitative concept introduced by Ralph Pearson to explain the stability of metal complexes and the mechanisms of their reactions.

However it is possible to quantify this concept based on Klopman's FMO analysis using interactions between HOMO and LUMO.

According to this theory, the Lewis acids and bases can be further divided into hard or soft or border line types.

Hard Lewis acids are characterized by small ionic radii, high positive charge, strongly solvated, empty orbitals in the valence shell and with high energy LUMOs.

Soft Lewis acids are characterized by large ionic radii, low positive charge, completely filled atomic orbitals and with low energy LUMOs.

Hard Lewis bases are characterized by small ionic radii, strongly solvated, highly electronegative, weakly polarizable and with high energy HOMOs.

Soft Lewis bases are characterized by large ionic radii, intermediate electronegativity, highly polarizable and with low energy HOMOs.

The Border line Lewis acids and bases have intermediate properties.

Remember that it is not necessary for Lewis acid or base to possess all the properties to be classified as hard or soft or borderline.

In short, Hard acids and bases are small and non-polarizable, whereas Soft acids and bases are larger and more polarizable. 

HSAB Principle: According to HSAB concept, hard acids prefer binding to the hard bases to give ionic complexes, whereas the soft acids prefer binding to soft bases to give covalent complexes. 

* The large electronegativity differences between hard acids and hard bases give rise to strong ionic interactions.

* The electronegativities of soft acids and soft bases are almost same and hence have less ionic interactions. i.e., the interactions between them are more covalent.

* The interactions between hard acid - soft base or soft acid - hard base are mostly polar covalent and tend to be more reactive or less stable. The polar covalent compounds readily form either more ionic or more covalent compounds if they are allowed to react.

According to FMO analysis, the interactions between acids and bases are controlled by the relative energies of the participating frontier molecular orbitals (FMO) i.e., HOMO and LUMO.



Hard acids * Atomic centres of small ionic radii (<90 pm).
* High positive charge.
* Empty orbitals in their valence shells.
* Low electronegativity (0.7-1.6) and low electron affinity.
* Likely to be strongly solvated.
* High energy LUMO.
H+, Li+, Na+, K+, Be2+, Mg2+, Ca2+, Sr2+, Sn2+

Al3+, Ga3+, In3+, Cr3+, Co3+, Fe3+, Ir3+, La3+, Si4+, Ti4+, Zr4+, Th4+, VO2+ , UO22+

BeMe2, BF3, BCl3, B(OR)3, AlMe3
Soft acids * Large radii (>90 pm).
* Low or partial positive charge.
* Completely filled orbitals in their valence shells.
* Intermediate electronegativities (1.9-2.5)
* Low energy LUMO's with large magnitude of LUMO coefficients.
Cu+, Ag+, Au+, Hg+ , Cs+ , Tl+ , Hg2+ , Pd2+, Cd2+ , Pt2+ 

Metal atoms in zero oxidation states
Border line acids   Fe2+ , Co2+ , Ni2+ , Cu2+ , Zn2+ , Pb2+ , B(CH3)3, SO2, NO+
Hard bases * Small radii (around 120pm) & highly solvated .
* electronegative atomic centres (3.0-4.0).
* Weakly polarizable.
* Difficult to be oxidized.
* High energy HOMO.
H2O, OH-, F-, Cl-, CH3CO2-, PO43-, SO42-, CO32-, NO3-, ClO4-, ROH, RO-, R2O, NH3, RNH2, N2H4
Soft bases * Large atoms (>170 pm) with intermediate electronegativity (2.5-3.0).
* High polarizability
* Easily undergo oxidation.
* Low energy HOMO's but large magnitude HOMO coefficients.
RSH, RS-, R2S, I-, CN-, SCN-, S2O3-, R3P, R3As (RO)3P, RNC, CO, C2H4, C6H6, R-, H-
Border line bases   Aniline, pyridine, N3-, Br-, NO2-, SO32-, N2


In hydrogen bonding: The strong hydrogen bond is possible in cases of  H2O, NH3 and HF, since the donor atoms (F, O & N) are hard lewi bases and their interactions with partially positively charged H, which is a hard acid, are stronger.


Linkage of ambidentate ligands to metal atoms: The ambidentate ligand, SCN- can bind either by S end or N end. The bonding mode can be determined by using HSAB principle. 

It bonds through sulfur atom (soft base) when bonded to Pt2+, a soft acid. 

However it bonds through nitrogen atom (a hard base) when linked to Cr3+, a hard acid. 


Site preference in organic reactions: RCOX is a hard acid and reacts with the nitrogen end of SCN- ion to form an acyl isothiocyanate.

Whereas the softer methyl group bonds to the Sulfur atom and forms methyl thiocyanate.

methyl as soft acid + soft basic sulfur end 


Inorganic reactions: HSAB principle is used to predict the outcome of some of the reactions. 

1) The following reaction is possible because As is softer than P and I- is softer than F-

More softer As prefers softer iodide over fluoride

Remember that both As and P are soft but relatively As is softer. 

2) The following reaction is possible since Mg2+ is harder acid than Ba2+ and O2- is harder base than S2-

harder acid Mg2+ combines with harder oxide ion


Precipitation reactions: The softer acids like Ag+, Hg+, Hg2+ etc., and border line acids like Fe2+, Ni2+, Cu2+, Zn2+, Pb2+ etc., can be precipitated as sulfides from their aqueous solutions since S2- ion is a softer base.


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Author: Aditya vardhan Vutturi