Chemistry Reference
In-Depth Information
(α' and β' are the dual basicity parameters of a ligand corresponding to X
and Y', respectively, and γ' is a constant determined for each ligand
considered.) This parameter was used in some QSAR studies (Enache
et al. 2003).
•
The Williams softness parameter is
W
, R or R (W)) is defined as the ratio of the
ionization potential to the ionic function, Z
2
/r where Z is the ionic charge and
r the ionic radius of the cation (Williams and Hale 1966; Williams et al. 1982):
=
Σ
I
=
Σ
Ir
x
.
(3.63)
R
Z
2
2
Z
r
Based on this relation, the behavior of class B metal ions in water can be
explained by the high polarizing power expressed by the ionization
potential of these cations, relative to their size and charge.
3.4.6 o
tHEr
p
aramEtErs
The covalent bond stability parameter (Δβ) was described as the difference between
the logarithm of the stability constants for the metal fluoride and the metal chloride
at infinite dilution (Turner et al. 1981):
Δβ = log β
o
MF
− log β
o
MCl
(3.64)
where Log β
o
MF
= Log of the stability constant for the metal fluoride and Log
β
o
MCl
= Log of the stability constant for the metal chloride. The tendency to form
covalent bonds with soft ligands decreases with Δβ.
The combination of Δβ and Z
2
/r was also used (Turner et al. 1981). Plotting Z
2
/r
(vertical axis) as a function of Δβ (horizontal axis), it was possible to create a com-
plexation field diagram for metal ions, where the stability constants of metal ion
complexes with intermediate ligands increases across the diagram (Walker et al.
2003).
3.5 BRÖNSTED ACIDITY OF METAL IONS
The metal ions are found in aqueous solution surrounded by water molecules as aqua
complexes (or aqua acids). The hydrated ions can undergo stepwise hydrolysis with
delivery of one or more H
+
to the bulk solvent. This process occurs due the attraction
of the metal ion to the electron cloud of its water hydration, and the weakening of
the OH bond in the H
2
O molecule. The process can be represented by the reaction,
MOH
2
n+
+ H
2
O = MOH
(n−1)+
+ H
3
O
+
.
(3.65)
Any positive ion can participate in this reaction to some extent, as expressed as the
equilibrium constant (K
a
) for the above equation.
