Biomedical Engineering Reference
In-Depth Information
TABLE 10.3
Classifi cation of Lewis Acids
Hard
Soft
H
+
, Li
+
, Na
+
, K
+
Cu
+
, Ag
+
, Au
+
, Tl
+
, Hg
2
2+
Mg
2+
, Ca
2+
, Sr
2+
, Mn
2+
Pd
2+
, Cd
2+
, Pt
2+
, Hg
2+
Al
3 +
, La
3+
, Gd
3+
, Cr
3+
, Co
3+
, Fe
3+
, As
3+
Tl
3 +
, Au
3+
Si
4+
, Ti
4+
, Os
4+
Borderline
Fe
2+
, Co
2+
, Ni
2+
, Cu
2+
, Zn
2+
, Sn
2+
, Pb
2+
, Sb
3+
, Bi
3+
, Ru
2+
, Os
2+
, NO
+
For “class a” metal ions, the order of complex stability is as follows, whereas for “class b” metal ions
the order is virtually the opposite.
F > C1 > Br > I
O > > S > Se > Te
N > > P > As > Sb > Bi
In Table 10.3, metal ions that exhibit some importance in the bioinorganic chemistry are
classii ed.
The general feature of a “class a” metal ions includes a small ionic radius, high positive charge,
and are called “hard (Lewis) acids.” “Class b” metal ions are in contrast associated with low oxida-
tion state, large ionic radius, and are called “soft (Lewis) acids.” This leads to a useful corollary,
which is as simple and useful:
“Hard acids prefer to coordinate to hard bases, while soft acids prefer soft bases”
The stability order for soft acid complexes with Lewis bases is as follows:
S~CI BrC1NOF
>>
> > > >
For hard acids, the division is even sharper since only complexes with oxygen or l uorine donor
atoms will exist in aqueous solution. The HSAB (hard and soft acid and base) principle will be
widely applied in the following text.
10.4.3 K
INETICS
. I
NERT
AND
L
ABILE
C
OMPLEXES
Any complex formation takes place in a substitution reaction by replacement of one ligand by
another. Thus, any substitution reaction is fundamentally a Lewis acid-base reaction.
The rate of a substitution reaction is primarily determined by the ratio between charge and size
(charge density) of the metal ion, but when transition metals are involved the
d
-electron structure
should also be taken into account. The term “labile” will be used for very reactive complexes while
less reactive ones are called inert. Care should be taken not to confuse the term “labile” (kinetic)
with the thermodynamic designation, stable.
Knowledge of the kinetic properties of complexes will obviously be decisive in the design of
drugs. If a pharmaceutical in the form of an organic molecule is transferred to a target site by
means of a metal ion, the complex should not be highly inert. On the other hand, complex formation
between Pt(II) and DNA bases should be sufi ciently inert in order to have adequate time to affect
the division of tumor cells.
The electronic structure of transition metal ion complexes determines their reactivity due to the
particular occupancy of the
d
-orbitals. Following the crystal i eld theory, the i ve
d
-orbitals split in
the presence of the electrostatic i eld provided by the ligands (crystal i eld).
The doubly degenerate energy levels are denominated
e
while the triply degenerate levels are
called
t
2
. The two high-energy
d
-orbitals of an octahedral complex are thus type
e
-orbitals while the
