Chemistry Reference
In-Depth Information
Inner-sphere electron transfer involves the inner coordination sphere of the metal
complexes, and normally takes place through a bridging ligand. A classic example
studied and explained by Taube (1953) is given in equation 3.55:
[CoCl(NH
3
)
5
]
2+
+ [Cr(H
2
O)
6
]
2+
→ [Co(NH
3
)
5
(H
2
O)]
2+
+ [CrCl(H
2
O)
5
]
2+
. (3.55)
In this reaction, the chloride that was initially bound to Co(III), the oxidant, becomes
bound to Cr(III) in complexes that are kinetically inert. The bimetallic complex
[Co(NH
3
)
5
(μ-Cl)(Cr(H
2
O)
5
]
4+
is formed as an intermediary, wherein “μ-Cl” indicates
the chloride bridges between the Cr and Co atoms, serving as a ligand for both. The
electron transfer occurs across a bridging group from Cr(II) to Co(III) to produce
Cr(III) and Co(II).
Redox changes occurring in a biological environment can have a pronounced
influence on the overall toxicological (biological) response elicited by a metallic
complex.
A typical example is mercury, which can exist in two oxidation states and the free
state. These species exhibit marked differences in uptake, distribution, and toxico-
logical effects. The three forms are governed by the following disproportion reaction
(Equation [3.56]):
Hg
2
2+
⇌ Hg
0
+ Hg
2+
(3.56)
The intrinsic lability/reactivity can be modified by redox changes in vivo. Thus,
an inert reactant can become a labile product and vice versa. The classical redox
reaction of Taube (Equation [3.54]) implies reduction of the inert Co(III) species by
the labile Cr(II), producing labile Co(II) and an inert Cr(III) species. Changing of
intrinsic lability/reactivity can lead to the stronger bonds between biological ligands
(e.g., nucleic acids) and very inert complexes. As a consequence, the metal might not
be removed easily by the normal substitution reactions or repair processes.
3.4
PROPERTIES OF METAL IONS RELEVANT TO IONIC
AND COVALENT BONDING TENDENCIES
3.4.1 i
ionic
p
otEntial
(Z/
r
)
The ionic potential (Z
/r
) of a metal ion is given by the charge/radius ratio (Z is ion
charge and
r
is ionic radius). It incorporates the distance between an ion and another
charge, and the size of the electrostatic force created. It reflects the metal ion ten-
dency to form ionic bonds.
3.4.2 t
HE
i
ionic
i
ndEx
(Z
2
/
r
)
The polarizing power (
Z
2
/
r
)
is a measure of electrostatic interaction strength between
a metal ion and a ligand (Turner et al. 1981). The quotient
Z
2
/
r
is a surrogate charac-
teristic indicating ionic bond stability.
