HSAB Theory - Theory

Theory

The gist of this theory is that soft acids react faster and form stronger bonds with soft bases, whereas hard acids react faster and form stronger bonds with hard bases, all other factors being equal. The classification in the original work was mostly based on equilibrium constants for reaction of two Lewis bases competing for a Lewis acid.

Hard acids and hard bases tend to have the following characteristics:

  • small atomic/ionic radius
  • high oxidation state
  • low polarizability
  • high electronegativity (bases)
  • hard bases have highest-occupied molecular orbitals (HOMO) of low energy, and hard acids have lowest-unoccupied molecular orbitals (LUMO) of high energy.

Examples of hard acids are: H+, light alkali ions (Li through K all have small ionic radius), Ti4+, Cr3+, Cr6+, BF3. Examples of hard bases are: OH–, F–, Cl–, NH3, CH3COO–, CO32–. The affinity of hard acids and hard bases for each other is mainly ionic in nature.

Soft acids and soft bases tend to have the following characteristics:

  • large atomic/ionic radius
  • low or zero oxidation state
  • high polarizability
  • low electronegativity
  • soft bases have HOMO of higher energy than hard bases, and soft acids have LUMO of lower energy than hard acids. (However the soft-base HOMO energies are still lower than the soft-acid LUMO energies.)

Examples of soft acids are: CH3Hg+, Pt2+, Pd2+, Ag+, Au+, Hg2+, Hg22+, Cd2+, BH3. Examples of soft bases are: H–, R3P, SCN–, I–. The affinity of soft acids and bases for each other is mainly covalent in nature.

Acids Bases
hard soft hard soft
Hydronium H+ Mercury CH3Hg+, Hg2+, Hg22+ Hydroxide OH- Hydride H-
Alkali metals Li+,Na+,K+ Platinum Pt2+ Alkoxide RO- Thiolate RS-
Titanium Ti4+ Palladium Pd2+ Halogens F-,Cl- Halogens I-
Chromium Cr3+,Cr6+ Silver Ag+ Ammonia NH3 Phosphine PR3
Boron trifluoride BF3 borane BH3 Carboxylate CH3COO- Thiocyanate SCN-
Carbocation R3C+ P-chloranil Carbonate CO32- carbon monoxide CO
bulk Metals M0 Hydrazine N2H4 Benzene C6H6
Gold Au+
Table 1. Hard and soft acids and bases

Borderline cases are also identified: borderline acids are trimethylborane, sulfur dioxide and ferrous Fe2+, cobalt Co2+ caesium Cs+ and lead Pb2+ cations. Borderline bases are: aniline, pyridine, nitrogen N2 and the azide, bromide, nitrate and sulfate anions.

Generally speaking, acids and bases interact and the most stable interactions are hard-hard (ionogenic character) and soft-soft (covalent character).

An attempt to quantify the 'softness' of a base consists in determining the equilibrium constant for the following equilibrium:

BH + CH3Hg+ ↔ H+ + CH3HgB

Where CH3Hg+ (methylmercury ion) is a very soft acid and H+ (proton) is a hard acid, which compete for B (the base to be classified).

Some examples illustrating the effectiveness of the theory:

  • Bulk metals are soft acids and are poisoned by soft bases such as phosphines and sulfides.
  • Hard solvents such as hydrogen fluoride, water and the protic solvents tend to solvate strong solute bases such as the fluorine anion and the oxygen anions. On the other hand dipolar aprotic solvents such as dimethyl sulfoxide and acetone are soft solvents with a preference for solvating large anions and soft bases.
  • In coordination chemistry soft-soft and hard-hard interactions exist between ligands and metal centers.

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