Chemistry Reference
In-Depth Information
-Cl
+OH
2
Co
Co
Co
OH
OH
OH
OH
2
Cl
cis
entry
cis
square pyramidal
intermediate
H
2
O
Co
trans
entry
OH
Cl
-Cl
Co
Co
+OH
2
OH
OH
cis
entry
Co
OH
trans
trigonal bipyramidal
intermediate
OH
2
Figure 5.12
Stereochemical control of the substitution reaction in
cis
-and
trans
-[CoCl(en)
2
(OH)]
+
via different
transition state geometries. The square pyramidal intermediate offers only one choice to the entering
group and 100% retention of geometry, the trigonal bipyramidal intermediate offers three entry points
around the triangular central plane, two leading to
cis
entry and one to
trans
entry, for a 2:1 ratio.
which is a simple hydrolysis substitution reaction proceeding (as for the vast majority of
cobalt(III) complexes) via a dissociative mechanism. In the D mechanism in Figure 5.9, two
different five-coordinate intermediates were suggested - trigonal bipyramidal and square-
based pyramidal. This reaction offers us the opportunity to both support this mechanistic
assertion and identify the likely geometry of the transition state. It does this by examining
the stereochemistry of the products. For
cis
-[CoCl(en)
2
(OH)]
+
reacting, there is 100% con-
version to
cis
-[Co(en)
2
(OH)(OH
2
)]
2
+
, whereas for
trans
-[CoCl(en)
2
(OH)]
+
reacting there
is
30%
trans
isomers of the product formed. Given the very close similarity
in the precursor complexes, a common gross dissociative mechanism is presumed - so why
the change in isomer ratio? The answer lies in the form of the transition state.
For a square-based pyramid, full retention of stereochemistry for the
cis
isomer is
predicted (Figure 5.12) as only one site of attachment is available for the incoming water
group to regenerate the original stereochemistry, whereas the trigonal bipyramidal shape
for the
trans
isomer allows choice, because water entry can occur from any of three
different positions around the triangular plane, some of which lead to different geometric
isomers (in fact, a theoretical ratio of 2:1
cis
:
trans
is predicted, not too distant from
the experimental results). Thus, simple experiments with geometric isomers, involving
separating and identifying product isomers, allows the reaction mechanism to be probed in
some detail.
∼
70%
cis
and
∼
5.3.1.1.5 Base Hydrolysis
Although most reactions we meet occur in aqueous solution, we have not yet looked at the
effect of one obvious variable in aqueous solution, which is pH change. Reactions may
occur in neutral, acidic or basic solution. What has been observed is that some reactions
are accelerated by protons and others by hydroxide ions. Such observations infer a role for
