7.13B: Hard and Soft Metal Centers and Ligands

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Hard and Soft Acids and Bases, Pearson's HSAB

This theory proposes 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 largely based on equilibrium constants for the 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)

Examples of hard acids are: H⁺, light alkali ions (Li through K are considered to have small ionic radii), Ti⁴⁺, Cr³⁺, Cr⁶⁺, BF₃. Examples of hard bases are: OH^-, F^-, Cl^-, NH₃, CH₃COO^-, CO₃^2-. 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 bonding
high polarizability
low electronegativity

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.

HSAB acids and bases This provides a qualitative approach to looking at the reactions of metal ions with various ligands since, from the diagram above, it is expected that whereas Al(III) and Ti(III) would prefer to react with O-species over S-species, the reverse would be predicted for Hg(II).