Chemistry Reference
In-Depth Information
DISSOCIATIVE
+C
[MA
5
B] B + [MA
5
]
#
[MA
5
C]
slow
step
#
A
A
A
M
reaction rate =
k
[MA
5
B]
A
+C
-B
B
C
A
A
A
A
A
A
A
five-coordinate
intermediate
slow
fast
M
M
A
A
#
A
A
A
-B
+C
A
AM
A
A
-B
[MA
5
B] + C [MA
5
BC]
#
[MA
5
C]
ASSOCIATIVE
slow
step
#
B
A
reaction rate =
k
[MA
5
B] [C]
A
C
M
A
A
-B
+C
B
C
A
A
A
A
A
A
A
seven-coordinate
intermediate
slow
fast
M
M
A
A
#
A
A
C
B
+C
-B
A
A
A
M
A
A
Figure 5.9
The operations involved in the dissociative and associative reaction mechanisms for octahedral
complexes. Two different shapes for intermediates are based on known shapes for these coordination
numbers.
in which the process of substitution of the ligand to be replaced (the leaving group) by a new
ligand (the entering group) occurs, as outlined above, and in Figure 5.9. The outcome is in
each case identical, of course - one ligand has been replaced by another. The mechanism
that operates cannot be observed directly, but must be inferred from various experiments.
The above discussion could suggest that a successful forward step always occurs. If en-
ergy acquired in an encounter or collision process between reacting molecules is insufficient
to allow the reactant to reach the energy level of the activated state, then no reaction can
occur; this is a key aspect of activation theory. Moreover, the transition state, when reached,
does offer a 'forward or back' option in both mechanisms. With reference to Figure 5.9,
should B re-enter the coordination sphere (in the D mechanism) or C depart before B (in the
A mechanism), then the initial reactant is regenerated. Thus there is a potential reversibility
in the reactions, although this has been neglected here for the sake of simplification. The
observation of change in a reaction tells us that there is a driving force towards the products.
5.3.1.1.1 Dissociative Mechanism
Slow and therefore rate-determining loss of one ligand to produce a
five-coordinate in-
termediate
is the key to this process. This intermediate is a short-lived transition state of
higher energy than the reactants; with energy (the activation energy) required to reach this
intermediate state, this is the defining slow (or rate determining) step in the overall reaction
