Information Technology Reference
In-Depth Information
acter. Moreover, the borderline classifi cation can easily distinguish the
acids or bases, for example, among the halides, fl uoride, and chloride are
“hard”; bromide is “borderline”; and iodide is “soft.” Pearson's hard-soft
classifi cation is very popular in the scientifi c community. For example,
Hudson [22] applied the HSAB concept to explain a wide variety of nu-
cleophilic displacement reactions.
Pearson [9] noted that “the bonding between hard acids and bases is
dominated by electrostatic interactions. This is shown by the fact that a
strong interaction correlates with (1) small size and (2) high electronega-
tivity of the base. These are purely electrostatic parameters, and this im-
plies that the bonding is essentially ionic” and “the bonding between soft
acids and bases seems to be primarily covalent in nature. Thus, a strong
interaction occurs between large metal ions with high electronegativity
and the most polarizable bases.”
The hardness and/or softness of the acids and bases depend on several
factors, such as the following:
1.
The electronegativity and the hardness of the acids and bases
2.
Number of donor and acceptor atoms and/or molecules
3.
Electrophilic and nucleophilic behavior of the acids and bases
4.
Polarity and polarizability of the acids and bases
5.
The group contributions for electronic, magnetic, thermodynamic,
and other properties (symbiosis)
SYMBIOSIS
In a landmark work, Jorgenson [23] pointed out that the hard and soft
behavior of an acidic site and basic site is not an inherent property of the
particular atom at that site, but the behavior can be influenced by the sub-
stituent atoms. The addition of soft, polarizable substituent(s) can soften
an otherwise hard center and the presence of electron withdrawing sub-
stituents can reduce the softness of a site. Jorgenson [23] referred this
tendency of a group or ligand (substituent) to increase or decrease the
hard-soft behavior of a center as “symbiosis.”
Thus, one the one hand, the hard metal ions have little electron density
to share with a ligand. Hard ligands do not readily give up their electron
density, thus the combination of the hard acid and hard base is stabilized
by simple electrostatic force of attraction
.
On the other hand, soft metal
ions and ligands are more prone to sharing electron density with a greater
Search WWH ::
Custom Search