HSAB theory

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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 in understanding 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.

[edit] History

Ralph Pearson introduced the Hard Soft Acid Base (HSAB) principle in the early 1960s ^{[1] [2]} as an attempt to unify inorganic and organic reaction chemistry.

[edit] 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⁺ ⇌ 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).

[edit] References