HSAB theory

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Ralph Pearson lecturing in Northwestern University in 1969.

The HSAB concept, also known as HSAB theory, is widely used in chemistry for explaining stability of compounds, reaction paths etc. HSAB is the acronym for hard and soft acids and bases 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.

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

History

Ralph Pearson introduced the Hard Soft Acid Base (HSAB) principle in the early 1960s as an attempt to unify inorganic and organic reaction chemistry. The impact of the new idea was immediate, however, since then the HSAB principle has fallen by the wayside while other approaches developed at the same time, such as frontier molecular orbital (FMO) theory and molecular mechanics, have flourished.

Kinds of acids and bases

Among the hard acids are: H⁺, alkali ions, Ti⁴⁺, Cr³⁺, Cr⁶⁺, BF₃, ...

Soft acids: CH₃Hg⁺, Pt⁴⁺, Pd²⁺, Ag⁺, Hg²⁺, Hg₂²⁺, Cd²⁺, BH₃, ...

Hard bases: OH^–, F^–, Cl^–, NH₃, CH₃COO^–, CO₃^2–, ...

Soft bases: H^–, R₃P, SCN^–, I^–, ...

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⁺ [\leftarrow \rightarrow] H⁺ + CH₃HgB

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

FMO Analysis

The general qualitative nature of HSAB being insufficient for many applications, a more specific definition based upon frontier molecular orbital theory was created by Gilles Klopman who proposed:

Hard [Lewis] acids bind to hard [Lewis] bases to give charge-controlled (ionic) complexes. Such interactions are dominated by the +/– charges on the Lewis acid and Lewis base species.

Soft [Lewis] acids bind to soft [Lewis] bases to give FMO-controlled (covalent) complexes. These interactions are dominated by the energies of the participating frontier molecular orbitals (FMO), the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).

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