THE TIME EVOLUTION OF THE HARD AND SOFT ACIDS AND BASES THEORY - Theoretical and Computational Research in the 21st Century - page 26

react as a monomer. In the extension of Eq. (16) to cation base systems, the
transfer energy is accommodated by adding an R A T A term:
Thus, Eq. (16) becomes

- ΔH = C A C B + - E A E B + R A T A (17)

where “the receptance” R A identifies the Lewis acid as the receptor of the
transferred electron density, as being related to the enthalpic stabilization
of the cation per increment of electron density transferred to it. The term
T B is called “the transmittance,” which determines the extent of the trans-
fer from the base and the product represents the enthalpic stabilization that
accompanies the electron transfer into the cation from the base. Thus, T B
relates to the ease of removing electron density from the base in the pres-
ence of the cation.
The four-parameter equation and its modifi ed versions seem to give
excellent agreement with experiment and compares very favorably with
that suggested by Pearson. However, its empirical nature and the number
of independent parameters involved in the calculations make it very im-
practical to use. In addition, no physical reason or explanation for hard and
soft behavior is provided by such an approach.

1.2.10 THE DENSITY FUNCTIONAL UNDERPINNING OF
THE HSAB PRINCIPLE

The HSAB principle remained qualitative for predicting the nucleophilic
displacement reactions for a very long time. Although the strengths of ac-
ids and bases can be adequately described in terms of the relative PK val-
ues of the acids and the bases within the Lowry-Brønsted acid basetheory,
there was no single scale of Lewis acidity and basicity and also there was
no measure of the acid-base strength and the ease of the reaction.
In 1980, Jenson [41] used a relation between second ionization poten-
tials versus z / r , where z is the atomic number and r is the radius of atom,
to classify divalent metal ions into three broad categories: hard, soft, and
borderline. Although this classifi cation was not that acceptable by scien-
tifi c community, this graphical classifi cation was used by Williums [42] as
an input of his work.
The scientifi c world had to wait till the seminal work of Parr and co-
workers who, using the DFT as basis, placed the qualitative HSAB principle