Chemistry Reference
In-Depth Information
pre-equilibrium
Cl
Cl
K
H
3
N
H
3
N
NH
3
NH
3
experimental
rate
constant
-H
+
Co
Co
+H
+
NH
3
NH
3
H
3
N
H
3
N
-
NH
2
NH
3
[
-
OH]
rate =
k
obsd
[CoCl(NH
3
)
5
2+
][
-
OH]
conjugate base
slow
(rate
determining
step)
k
2
Rate derived from the mechanism:
OH
NH
3
Kk
2
[CoCl(NH
3
)
5
2+
][
-
OH]
1 + K[
-
OH]
k
obsd
[CoCl(NH
3
)
5
2+
][
-
OH]
H
3
N
rate =
NH
3
NH
3
fast
+ OH
2
H
2
N
-
+
Cl
-
Co
Co
NH
3
NH
3
H
3
N
=
for
K
[
-
OH] >> 1, and
k
obsd
=
Kk
2
NH
3
NH
3
five-coordinate intermediate
Figure 5.13
Mechanism for base hydrolysis of cobalt(III) complexes. A pre-equilibrium ammine deprotonation
step is followed by slow loss of the leaving group (Cl
−
here) to form a short-lived intermediate that
reacts readily with solvent water to form the hydrolysed product. A first-order dependence on [
−
OH]
is observed experimentally (top inset), with the mechanism yielding the equivalent expression (lower
inset).
these ions in the mechanism. We shall examine the origin of such effects in one case only
here, with hydroxide ions.
Reactions in aqueous solution in the presence of added base (hydroxide ion) may lead
to substitution of one donor group by hydroxide, a process termed
base hydrolysis
.As
some ligands are not readily replaced by hydroxide ion, the leaving group is not selected
arbitrarily. Some ligands are more susceptible to reactions than others. A simple example
is the complex ion [CoCl(NH
3
)
5
]
2
+
, where the NH
3
groups are only very slowly replaced,
whereas the Cl
−
ion is much more readily removed, leading to a specific reaction (5.40).
[CoCl(NH
3
)
5
]
2
+
+
−
OH
[Co(NH
3
)
5
OH]
2
+
+
Cl
−
→
(5.40)
The reaction is usually catalysed by base, seen experimentally by the reaction getting
faster as the concentration of base increases. For this behaviour, Equation (5.41) fits the
experimental behaviour.
k
obsd
[CoCl(NH
3
)
2
5
][
−
OH]
rate
=
(5.41)
Upon initial inspection, this expression looks like that given earlier for an A mechanism,
but is this the case? There is strong support for a separate, distinct mechanism in this case,
involving
conjugate base
formation. From a range of studies, the now accepted mechanism
involves a pre-equilibrium deprotonation and a following D mechanism, as illustrated in
Figure 5.13.
The process involves formation of the conjugate base of the complex by proton loss from
an ammonia ligand that remains intact in the complex, this amide ion being reprotonated
to regenerate the ammonia in the fast final step where the intermediate reacts with addition
