We
investigate the physical basis, validity, and limitations of
the minimum electrophilicity principle, MEP, which postulates that
the sum of the electrophilicity indices, ∑ω, of the reaction
products will be smaller than that of the reactants, Δω
< 0. We present a much-improved understanding of the conditions
for minimizing electrophilicity indices. Two indices, ω1 = (I + A)2/8(I – A) and ω2 = I·A/(I – A), are discussed, using ionization energies, I, and electron affinities, A, obtained from either
ground-state (GS) or valence-state (VS) energies. The performances
of ω1 and ω2 are compared for a
wide range of chemical species from diatomic molecules, through large
clusters to liquid water and solid crystals. New analytical arguments
in support of MEP are found. Two new theorems are proved, and three
new rules rationalize the changes Δω1 and Δω2 in association reactions, X + Y → XY. They explain
why MEP is much more successful as a guiding rule than the maximum
hardness postulate in such reactions. On the other hand, they also
identify the increased electron affinity of the product as the reason
for the rare but highly significant failures of MEP, e.g., in B2, C2, Si2, and CN. As a rule, electrophilicity
is minimized in association reactions. However, both ω1 and ω2 are increased if the bond dissociation energy D(XY–) is larger than D(XY), which is equivalent to an increased product electron affinity.
The large positive changes Δω1 and Δω2 in 2C → C2 exhibit a strong contrast to
MEP. The changes in electrophilicity indices may help gain insights
into the versatility of the chemistries of carbon and other elements.
Solid-state double-exchange reactions are correctly assessed by Kaya’s
composite descriptor, somewhat less by ω2, but not
at all by ω1. A wide class of failures of MEP is
found as size-driven electrophilicity maximization, Δω
> 0, e.g., in fullerenes, large metal clusters, and liquid water.
Many electrophiles, especially superelectrophiles, show significantly
larger electrophilicity indices than the largest index of their isolated
atoms. The changes Δω1 and Δω2 provide important information on the reactivities of chemical
systems; however, it appears that the minimum electrophilicity postulate
cannot serve as a basis for a theory.
