Chemistry Reference
In-Depth Information
symmetrical
short-lived
intermediate
D
#
D
D
D
D
D
D
D
dissociative
mechanism
D
D
M
M
M
D
D
D
D
D
D
D
D
Λ
∆
#
D
D
D
D
D
D
D
-L
D
D
+L
associative
mechanism
L
M
M
M
D
D
D
D
D
+L
-L
D
D
D
D
Λ
∆
D
D
#
D
D
D
D
D
D
D
trigonal twist
mechanism
M
M
M
D
D
D
D
D
D
D
D
Λ
∆
Figure 5.17
Mechanisms for isomerization of an octahedral complex with three didentate chelates, proceeding
through a symmetrical transition state.
For some complexes, the isomerization is very slow (as for the inert cobalt(III) complex
[Co(en)
3
]
3
+
), whereas for others the process is sufficiently fast (as for the nickel(II) analogue
[Ni(en)
3
]
3
+
) that the complex cannot be easily resolved into its optical forms through
conventional crystallization methods. The isomerization reaction can be readily followed
by observing the loss of optical rotation at a selected wavelength over time; this is usually
a simple first order exponential decay process.
5.3.3
A New Face - Oxidation-Reduction
Oxidation-reduction (electron transfer) reactions are important in chemistry and biology.
When a chemical oxidation of
A
by
B
occurs,
B
itself is reduced - an
electron transfer
process has occurred. For such chemical processes, there is always a partnership between
an oxidant (which is reduced in carrying out its task) and a reductant (which is oxidized
in the reaction); thus we frequently talk of oxidation-reduction, or (for ease of use) redox,
reactions for what are essentially electron transfer processes. Of course, an electron can be
