THE TIME EVOLUTION OF THE HARD AND SOFT ACIDS AND BASES THEORY - Theoretical and Computational Research in the 21st Century - page 21

strong electron transfer occurs between them. Klopman [24] called the
reaction as a “frontier-controlled reaction . ”
It occurs only in reactions between nucleophiles of low electronegativ-
ity and electrophiles of high electronegativity with a good overlap of the
interacting orbitals. Such an effect refl ects to covalent bonding and can be
associated with soft-soft interaction (Figure 1.2).

FIGURE 1.2 Frontier-control effect.

In brief, Klopman [24(b)] made an attempt to put the qualitative HSAB
theory on a sound frontier orbital theory (FMO) and classifi ed the do-
nor and acceptor molecules into hard and soft categories on the basis of
the difference in energy between the highest-occupied orbital of the nu-
cleophile (donor), Ψ m , and the lowest empty orbital of the electrophile or
acceptor, Ψ n , that is, the frontier orbitals. Klopman noticed that the hard
acceptors will tend to form complex with the hard donors and soft accep-
tors will prefer soft donors. Thus, Klopman tried to justify the statement
of Pearson in terms of frontier orbitals of the interacting molecules; hence,
his paradigm was labeled as FMO theory of HSAB principle. Klopman
[24(b)] concluded that “ hard-hard interactions are charge controlled and
depend mainly on the ionic interaction of the reagents, in addition to that
the steric inhibition of solvation which may occur during the union of the
ions further accentuates the endothermicity of the reaction characteristic
of hard-hard interaction. On the other hand, in the case of soft-soft inter-
action are counterbalanced by the stabilization brought about by covalent
bonding, and the reaction thus becomes exothermic.”