Chemistry Reference
In-Depth Information
complex may be reduced by transfer of an electron from an electrode to the complex;
oxidation can occur by transfer of an electron to an electrode. What is perhaps apparent
is that, for a solid electrode and a solution of a complex, it is only near the surface of the
electrode that the process can occur efficiently, so both the rate of transport to an electrode
surface, as well as the rate at which an electron is transferred between solid surface and
dissolved complex ion, are of relevance. In general, we can write a simple expression for
the process (represented here for reduction) (Equation 5.61):
[ML
m
]
n
+
+
e
−
→
[ML
m
]
(n
−
1)
+
(5.61)
The oxidation and reduction potentials of complexes are usually probed by the experimental
methods of
voltammetry
, which is a sensing rather than a complete conversion technique;
the full reduction or oxidation of a complex is achieved by
coulometry
, using large surface
area electrodes and stirring to enhance mass transport and thus speed up the process. These
techniques and their application are beyond the scope of this textbook.
Yet another, and perhaps more exotic, method of supplying an electron in solution is to use
radiolysis, where a very high energy source is directed into an aqueous solution to generate
a range of products from reaction with the abundant water molecules, with the aquated
electron (e
−
aq
) and the hydroxyl radical (
·
OH) being dominant species, and of particular
importance. The former is a powerful one-electron reductant, the latter a powerful one-
electron oxidant. The technique requires the use of high energy devices like a van der Graaf
generator or synchrotron, so is not widely available. However, the powerful radical species
formed can initiate definitive one-electron reduction or oxidation of complexes, either at
the metal centre (direct reduction or oxidation) or at the ligand (with metal reduction or
oxidation via a following intramolecular electron transfer possible). The former process
is a form of outer sphere reduction, with e
−
aq
as the reductant here. The latter process is
followed by electron transfer from the ligand radical to the metal ion, which is, in effect,
'half' of the reaction in the electron transfer through a bridge in a chemical inner sphere
process. When the high energy source is pulsed (the technique is called
pulse radiolysis
), a
reaction in solution can be initiated rapidly and the outcome followed kinetically. Electron
transfer reactions between two metal complexes can be initiated by pulse radiolysis also
under certain circumstances. For example, if a relatively high concentration of Zn(II) and a
low concentration of a Cr(III) complex are both present in solution, creation of e
−
aq
causes
it almost exclusively to react rapidly with the Zn(II), because it is in such very large excess,
to form the extremely rare and unstable Zn(I) ion. Then two reaction can occur with the
highly reducing Zn(I), in competition:
intermolecular electron transfer
between Zn(I) and
the Cr(III) complex; and
disproportionation
of Zn(I) to form equal amounts of Zn(II) and
Zn(0). By examining colour change associated with the chromium centre, the intermolecular
reaction can be examined. However, this technique is not one you are likely to meet often.
5.3.4
A New Suit - Ligand-centred Reactions
Changing ligands is a common feature of reactions that we have discussed in the section
above. However, it is possible to chemically alter a ligand while it remains attached to the
metal centre. Coordination need not prohibit chemistry going on with the bound ligand; in
fact, it may promote reaction as a result of electronic and positional influences resulting
from coordination. Because reactions of coordinated ligands is a topic intimately tied up
with synthesis, since new stable complex molecules are formed and from which new ligands
can be isolated, it is addressed in detail in Chapter 6.
