The maximum hardness principle implies the hard/soft acid/base rule

A recent paper [P. W. Ayers, J. Chem. Phys122, 141102 (2005)] considered the hard/soft acid/base exchange reaction, showing that the products associated with the hard/soft acid/base rule (in which the hard acid and hard base are bound, as are the soft acid and soft base) have lower energy than the alternative (in which the hard acid and soft base would have been bound and similarly the soft acid and hard base). Here we show that the maximum hardness principle also predicts this result. Unlike the previous derivation, we do not need to make any assumptions about the relative strength of the acids and bases.