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

The maximum hardness principle tool in relation to charge-transfer energy was investigated. Based on a consideration of charge-transfer energies, it was proved that the hard/soft acid/base exchange reaction is exothermic. Minimizing the usual energy expression with respect to the extent of charge transfer, the reaction energy was determined. The results show that the softest acid will always bind to the softest base available, leaving the harder base to bind to the harder acid.