What is the Hard Acid Soft Base Theory?

Hard Acid Soft Base Theory (HSAB):

• This theory works on the principle that soft acid reacts with the soft base while hard acid reacts with the hard base.
• HSAB is widely used for explaining the stability of compounds/molecules and the reaction mechanisms as well as pathways.
• In this theory, “Hard” stands for small species that have high charge states while “Soft” stands for big species with low charge states.

Examples of Hard and Soft acid/base:

• The hard acids can be ${\mathrm{H}}^{+}$and ${\mathrm{Li}}^{+}$ while the hard bases can be ${\mathrm{OH}}^{-}$ and ${\mathrm{F}}^{-}$.
• The soft acids are ${\mathrm{Cu}}^{+}$and ${\mathrm{Ag}}^{+}$while the soft bases are ${\mathrm{CN}}^{-}$ and ${\mathrm{I}}^{-}$