Tuning Reactivity of Platinum(II) Complexes

In a dissociation mechanism the transition state consists of a 14-electron (T-shaped)
intermediate of lower coordination number. The intermediate can last for a sufficiently
long time, allowing it to equilibrate with its environment before capturing the incoming
ligand. Accordingly, the rate of consumption of the incoming group is dependent on the
nature of the leaving group but is independent of the nature and concentration of the
incoming ligand. 7 Also, the consumption of the intermediate may occur when the leaving
group is still in close proximity to the metal centre since the bond length between the
metal centre and the leaving group (M---X) increases. 15 This reduces the ability of the
metal complex to discriminate and enables the solvent (usually present in excess) to
dominate the substitution process. In addition, contact times are long enough due to
high stability of the three coordinate intermediate which leads to ‘cis-trans
isomerisation’, thereby resulting in non-stereo specific products on addition of the
nucleophile. 5 For that reason, the dissociative mechanism is “non-stereo selective”.
2.2.2 The Associative Mechanism (A)
The intermediate is formed via two transition states (Figure 2.2), in which the bond-
making (M---Y) is more important than the bond-breaking (M---X) process as illustrated
in Scheme 2.1 and by Equation 2.2.
L2 Y
L3 Y
L2 bond
making
L3 L2 M
X
M bond
breaking
X
M
Y
L 1
entering group can attack from top or
bottom since sq.planar is easy to get into
leaving group
L1 trp transition state
Stereospecific-leaving group (X) is trans to L 2 and so is Y (entering group)
Scheme 2.1: Reaction pathway for the associative substitution reaction showing direct
nucleophilic attack
5
L 1
L 3
+ X