Chemistry Reference
In-Depth Information
Key experiments by Nobel laureate Henry Taube and co-workers, involving complexes
where chloride ion acts as a bridging ligand, cemented the mechanism. One reaction probed
was the classical reaction mentioned earlier (Equation 5.59):
5(NH
4
+
)
(5.59)
which is similar to the thiocyanate-bridged reaction discussed above, but with chloride ion
as the potential bridging group. The study made use of chloride ion introduced as an enriched
isotope, rather than as the natural isotopic mixture; by following the fate of the isotope in the
reaction, mechanistic information could readily be gleaned. Using isotopic (or 'labelled')
chloride ion (
36
Cl), it was found that the original cobalt-bound halide transferred completely
from precursor to the chromium product. If the labelled chloride was on the cobalt initially,
it was transferred fully to the chromium product, even in the presence of added natural-
isotope chloride ion in solution; conversely, using unlabelled cobalt-bound chloride and
labelled free chloride in solution, labelled chloride is not introduced into the chromium
product. These experiments effectively require a 'tight' interaction between oxidant and
reductant, best viewed as one where an intermediate that shares the transferring ligand
exists, that is
[Co
III
Cl(NH
3
)
5
]
2
+
+
[Cr
II
(OH
2
)
6
]
2
+
(
5H
+
)
[Cr
III
Cl(OH
2
)
5
]
2
+
+
Co
2
+
aq
+
+
→
Cr(OH
2
)
5
]
4
+
}
, which means an
inner sphere
mechanism.
The process of bridge formation can sometimes lead to a marked acceleration of the
electron transfer process compared with outer sphere reactions of related compounds. In
general, the key requirements for an inner sphere mechanism to operate can be summarized
as follows:
[(H
3
N)
5
Co
Cl
{
(a) one reactant must possess at least one ligand capable of binding simultaneously to two
metal ions in a bridging arrangement (this reactant is often the oxidant); and
(b) at least one ligand of one reactant must be capable of being replaced by a bridging
ligand in a facile substitution process (this replaced ligand in many examples is found
on the reductant).
The latter requirement means a relatively labile site must be available, and often involves
a coordinated water group. Note that atom transfer is
not
a requirement in this mechanism.
However, it can occur, and its occurrence is usually good evidence for the mechanism. For
example, when atom transfer occurs with an ambidentate ligand (like SCN
−
), the donor
preferred and bound by the precursor metal ion is often not the one attached to the stable
product, as described earlier; this helps define the mechanism. Nevertheless, the bridging
ligand
may
remain with its parent metal ion.
The type of bridging ligand can affect the observed electron transfer rate markedly. Its
role is to bring the metal ions together, and mediate the transfer through itself. For the
reaction discussed above but extended to employ a range of halide ions (Equation 5.60),
the rate increases with increasing size of the halide ion in the ratio for F : Cl : Br : I of 2 : 6
: 14 : 30.
[Cr
III
(OH
2
)
5
X]
2
+
(5.60)
This is assigned to increasing polarizability with increasing halide ion size. The higher-
charged Co(III) attracts more halide electron density towards its side of the bridging unit,
depleting the side near the Cr(II) from where the electron departs and thus facilitating
attraction of the transferring electron to the bridging ligand.
[Co
III
(NH
3
)
5
X]
2
+
+
[Cr
II
(OH
2
)
6
]
2
+
→
[Co
II
(NH
3
)
5
(OH
2
)]
2
+
+
