Electronegativity, acids, and bases—III Calculation of energies associated with some hard and soft acid-base interactions

Hard and soft acid-base interactions are investigated from the point of view of various factors contributing to the total bond energy. A method of estimating the total energy as the sum of three contributing energies is proposed. The three energies are the Madelung energy, derived from a coulombic model; the covalent energy, related to Pauling's geometric mean but allowing for loss of overlap with increasing ionicity; the electronegativity energy, the energy resulting from transfer of electron density from a less electronegative element to a more electronegative element. The latter is estimated from Mulliken-Jaffé electronegativies. The results correlate quite well (r = 0·98) with experimental enthalpies of atomization. It is concluded that hard-hard interactions contain considerable covalent bonding as well as Madelung energies. Fajans' rules and covalency, the “anomalous” bond energies of sodium and magnesium compounds, and the validity of the principle of electronegativity equalization are discussed.