Polarizability and Chemical Softness Parameter

A widely applied concept in the evaluation of metal ion interaction is that of hardness and softness. In general terms, hard and soft suggest the resistance to deformation in response to electric forces. Thus, hard ions have greater resistance to deformation of the electron cloud and soft ions have lesser resistance to deformation. Quantitative scales for metal ion hardness or softness were developed in the 1960s, starting with the HSAB theory developed by Pearson (see Section 3.3.1). 

Klopman attempted to quantify Pearson s HSAB principle using frontier molecular orbital (FMO) theory (Klopman 1968), with the following equation  

AE=energy change of interaction of species r with species s, 

Q=total initial charges, r=Coulomb repulsion term, 

n=energy of occupied frontier molecular orbital m, and E =energy of unoccupied frontier molecular orbital n. 

---