THE TIME EVOLUTION OF THE HARD AND SOFT ACIDS AND BASES THEORY - Theoretical and Computational Research in the 21st Century - page 8

there was no such theory that could explain the reactivity of such acids
and bases.
The hard-soft concept was born nearly a half-century ago with the re-
alization by a number of investigators that the strength of an acid or base
is dependent on several factors—extrinsic to intrinsic. Prior to the seminal
work of Pearson [9], the pioneering contribution to HSAB concept was
made by Ahraland, Chatt, Davies, Schwarzenbach, Jorgenson, Hudson,
Klopman, and Saville [12].
1.2.2 ORIGIN OF HARD AND SOFT CLASSIFICATION
In the earliest days of coordination chemistry, some basic trends concern-
ing the stability of complexes in certain ligand-central atom combinations
were discovered [13-16]. The main factors that affect the stability of com-
plexes in the solution were broadly classified into the following two main
types:
(i) Enthalpic effects: This effect includes the variation of bond
strength, steric and electrostatic repulsion between ligands in the
complex, disolvation enthalpy of the metallic ion and ligands when
the complex is formed, the conformation of the noncoordinated
and the coordinated ligands, charge neutralization of the positive
and negative ions; etc.
(ii) Entropic effects: This effect includes the number and size of the
chelate ring, number of the metal ions and ligands in solution,
changes in solvation in the formation of the complex, entropy
variations in noncoordinated ligands, changes in solvation in the
formation of the complex, configuration entropy differences of the
ligands in the complex; etc.
1.2.3 IRVING AND WILLIAM STABILITY SERIES
The idea of the hard-soft classification was initiated by Irving and Wil-
liam [17] who assigned the relative stabilities of complexes formed by
the metal ions with the ligands in the form of stability series. Based on
the stability of the products of the metal ions with ligands, they classified
metal cations into two classes. As a consequence, ligands were also classi-
fied into two types on the basis of reactions with the two types of cations.