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HSAB theory


Dec 15, 2021

HSAB concept is an initialism for “hard and soft (Lewis) acids and bases“. Also known as the Pearson acid-base concept, HSAB is widely used in chemistry for explaining stability of compounds, reaction mechanisms and pathways. It assigns the terms ‘hard’ or ‘soft’, and ‘acid’ or ‘base’ to chemical species. ‘Hard’ applies to species which are small, have high charge states (the charge criterion applies mainly to acids, to a lesser extent to bases), and are weakly polarizable. ‘Soft’ applies to species which are big, have low charge states and are strongly polarizable.[1]

Chemical theory about acids and bases

The theory is used in contexts where a qualitative, rather than quantitative, description would help in understanding the predominant factors which drive chemical properties and reactions. This is especially so in transition metalchemistry, where numerous experiments have been done to determine the relative ordering of ligands and transition metal ions in terms of their hardness and softness.

HSAB theory is also useful in predicting the products of metathesis reactions. In 2005 it was shown that even the sensitivity and performance of explosive materials can be explained on basis of HSAB theory.[2]

Ralph Pearson introduced the HSAB principle in the early 1960s[3][4][5] as an attempt to unify inorganic and organic reaction chemistry.[6]

. . . HSAB theory . . .

Hard–soft trends for acids and bases

Essentially, the theory states 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.[7] The classification in the original work was mostly based on equilibrium constants for reaction of two Lewis bases competing for a Lewis acid. [citation needed]

Comparing tendencies of hard acids and bases vs. soft acids and bases
Property Hard acids and bases Soft acids and bases
atomic/ionic radius small large
oxidation state high low or zero
polarizability low high
electronegativity (bases) high low
HOMO energy of bases[7][8] low higher
LUMO energy of acids[7][8] high lower (but > soft-base HOMO)
affinity ionic bonding covalent bonding
Examples of hard and soft acids and bases
Acids Bases
hard soft hard soft
Hydronium H3O+ 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 C6Cl4O2 Carbonate CO32− Carbon monoxide CO
Lanthanides Ln3+ Bulk metals M0 Hydrazine N2H4 Benzene C6H6
Thorium, uranium Th4+, U4+ Gold Au+

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, chloride, 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:

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. . . HSAB theory . . .