Tuning Reactivity of Platinum(II) Complexes

2.2.3 The Interchange Mechanism (I)
Between the limiting dissociation (D) and the limiting association (A) mechanisms
there exists a continuum of mechanisms that are characterised by a single activated
complex in which bond-making and bond-breaking between the metal centre and the
entering-plus-the leaving groups are synchronised. If the bond between the metal and
the leaving group is weakened before the incoming group tightly binds to the metal
centre, then the probability of the solvent attaching to the reactive metal centre is higher
leading to a dissociation mechanism. 7 This permits the solvent, which is normally in a
higher molar excess to dominate the substitution process, leading to a dissociative-
activated interchange mechanism (ID). If the reaction rate is more dependent on the
nature of the incoming group and the leaving group leaves the metal centre only once
the incoming group is fully bound to the reactive centre, the mode of activation changes
from ID to associatively-activated interchange mechanism (IA). 5
2.3 Measurements of Integrated Rate Constants
2.3.1 Reversible Second-order Reactions
Often, ligand substitution reactions in square-planar complexes do not go to completion,
but have a tendency to attain a state of equilibrium and can be written as:
M L 3 X + Y
A + B
k2 k-2 C
or
k 2
k -2
M L 3 Y + X
w here, A = M etal com plex (M L 3 X), X is the leaving group
B = incom ing nucleophile, Y
7
(2.4)
The forward reaction step is thus second-order, while the reverse reaction step is first-
order, which results in mixed order dependence. Because of the complex nature of the
reaction, the reaction can be studied by selecting pseudo first-order conditions, in which