Chemistry Reference
In-Depth Information
electron transfer
e
-
CN
CN
Cl
Cl
III
NC
II
CN
Cl
IV
Cl
NC
III
CN
Cl
Cl
Fe
Fe
Ir
Ir
+
+
NC
CN
Cl
Cl
NC
CN
Cl
Cl
CN
Cl
CN
Cl
Figure 5.18
A reaction involving electron transfer alone. The Fe(II) centre is oxidized to Fe(III) and the Ir(IV)
centre is reduced to Ir(III), without any change in the ligand set for either complex.
supplied or accepted electrochemically via an electrode rather than through using a chemical
reducing or oxidizing agent, and this is another way to initiate change in the oxidation state
of complexes. Here we shall concentrate mainly on chemically based systems, where an
oxidant and reductant of appropriate potentials need to be combined. A reaction will be
favourable if
E
0
0, with
E
0
being the difference between the standard potentials for the
two half-reactions (one for the oxidation part, one for the reduction) that are combined to
form the overall reaction.
There are two processes that are important in inorganic redox reactions. The key pro-
cess is, of course,
electron transfer
. However, some reactions also involve
atom transfer
,
whereby a component of one reacting molecule is transferred to another during the reac-
tion. It is not essential for both to occur, and many reactions are purely electron transfer
reactions. A classical example of a pure electron transfer alone is the reaction shown in
Figure 5.18.
This reaction of the two octahedral complexes occurs without any change in the coordi-
nation spheres, or ligand sets, of either metal. However, if you inspect the two metal centres,
you will note that the iron complex (the reductant) is oxidized from Fe(II) to Fe(III) and
at the same time the iridium complex (the oxidant) is reduced from Ir(IV) to Ir(III) - an
electron has been transferred from one metal to the other. This reaction can be conveniently
followed, since the colour of each species changes as it is converted from one oxidation
state to another.
Atom transfer alone can also occur, although it is a more difficult concept to come to grips
with. Think of it as an atom moving with its normal complement of valence electrons, and
no others, from one central atom to another.
Oxidative addition reactions
in organometallic
chemistry can be considered as a form of atom transfer. A typical example of this type is
given in Equation 5.53.
[IrCl(CO)(PR
3
)
2
]
+
Cl
2
→
[IrCl
3
(CO)(PR
3
)
2
]
(5.53)
Some reactions occur with both electron transfer and atom transfer. A simple example
of this class is the reaction of two octahedral complexes shown in Figure 5.19.
In the above example, an electron is transferred from the Cr(II) to the Co(III) and a
chloride ion is transferred from the cobalt to the chromium ion as well. The Cr(III) product
is inert, allowing it to be isolated and identified, thereby defining the presence of the
chloride ion in its coordination sphere. Colour changes in this reaction allow the reaction
to be readily monitored spectrophotometrically.
