THE TIME EVOLUTION OF THE HARD AND SOFT ACIDS AND BASES THEORY - Theoretical and Computational Research in the 21st Century - page 11

acceptors of class (b) are less numerous. They are almost all derived from
a number of neighboring elements occupying an area of more or less trian-
gular shape in the periodic table. The base of this triangle stretches, in the
sixth period, from about tungsten to polonium and its apex is at copper, Cu
(I) being a defi nite class (b) acceptor and Cu (II) on the border between the
classes. The most prominent class (b) acceptors are formed by elements
in the central part of this area. Generally, the class (b) acceptors include
heavier transition-metal cations in lower oxidation states, for example,
Cu + , Ag + , Cd 2+ , Hg + , Ni 2+ , Pd 2+ , Pt 2+ , etc. The class (b) acceptors form their
most stable complexes with the last ligand of each group. Thus, the affi nity
of class (b) acceptors toward the donors follows the following order:
(i) For group 15: N < P < As < Sb < Bi
(ii) For group 16: O < S < Se < Te
(iii) For group 17: F < Cl < Br < I
Ahrland et al. [19] classifi ed the donor ligands as class (a) or class (b)
depending on whether they formed more stable complexes with class (a)
or class (b) metals.
From this analysis, a principle can be derived as class (a) acceptors
prefer to bind to class (a) donors or ligands, and class (b) acceptors prefer
to bind to class (b) donors or ligands.
Ahrland and his coworkers proposed Π-bonding theory to explain the
metal ligand interactions [19].
1.2.5 PEARSON'S “HARD” AND “SOFT” CLASSIFICATION
OF “ACIDS” AND “BASES”
It is widely known that the HSAB concept of Pearson [9] is an acronym
for hard-soft Lewis acids and Lewis bases. The concept is widely used
in chemistry for explaining the stability of compounds, reaction mecha-
nisms, and reaction pathways. The term “hard” applies to the chemical
species that are small, have high charge states, and are weakly polarizable.
The term “soft” applies to the chemical species that are big, have low-
charge states, and are strongly “polarizable”.
The general idea of Ahrland et al. [19] was extended by Pearson [9]
to explain the general acid-base reactions. In 1962, Edwards and Pearson
[20] noticed that some substrates such as acyl halides or phosphate esters
react rapidly with strongly basic nucleophiles, such as OH - , but not with