A comprehensive analysis is presented for the acid-base double-exchange reaction as well as the associated acid-displacement and base-displacement “half-reactions” with the goal of elucidating the meaning of the hard/soft acid/base (HSAB) principle and the conditions for its validity. When electron-transfer effects are important and other effects are negligible, the HSAB principle is driven by the surpassing stability of the soft acid/soft base product. When electrostatic effects dominate the reactivity, the HSAB principle is driven by the surpassing stability of the hard acid/hard base product. Because electron-transfer effects favor soft/soft interactions, while electrostatic effects favor hard/hard interactions, acid-base exchange reactions may be used to determine whether a reagent’s reactivity is dominated by electron-transfer or by electrostatic effects. Because electron-transfer and electrostatic considerations separately favor the HSAB principle whenever the electronic chemical potentials of the acids and bases involved in the reaction are similar, our analysis provides strong support for the HSAB principle. The electronic chemical potential measures the intrinsic strength of acids and bases.
REFERENCES
This seems to be a reasonable assumption. On theoretical grounds, because the ionization potential of a reagent is usually substantially larger than its electron affinity, the value of the ionization potential makes the dominant contribution to both the chemical potential and the hardness. Empirically, both the electronegativity and the hardness increase as the oxidation state of a metal increases.
