Pearson′s chemical hardness, heterolytic dissociative version of Pauling′s bond-energy equation and a novel approach towards understanding Pearson′s hard–soft acid–base principle

Abstract

The empirical chemical hardness parameters developed recently by Pearson have been examined critically. It was found that these parameters normally characterise a group but not the corresponding ion as postulated. Equation (i) has been found quite successful when the heterolytic dissociation of D(A⁺B^–)–½[D(A⁺A^–)+D(B⁺B^–)]= 2|Δγ|²(i) AB into A⁺+ B^– follows the general notion of electronegativity (i.e. the more electronegative part of AB dissociates as the anion), where D(A⁺B^–) is the energy required to dissociate the AB bond into A⁺+ B^– and D(A⁺A^–) and D(B⁺B^–) are the heterolytic bond-dissociation energies of the molecules AA and BB respectively. Equation (i) has been used to define a new parameter γ, where |Δγ|=|γ_A⁺–γ_B^–|. The γ parameters characterise the ions, and have been used to rank various monovalent anions and cations in a unified way to explain the hard–soft acid–base principle of Pearson. It was found that a reaction of type (II) proceeds from left to right in the gas phase if the AB + CD → AC + BD (ii)|ΔΔγ| value of the right-hand side is greater than that of the left. Out of some 282 gas-phase reactions considered, there are few exceptions. Emphasis has been placed on explaining the course of inorganic reactions. It is felt that the γ parameter can be identified with the chemical hardness of an ion.