Tuning Reactivity of Platinum(II) Complexes

Potential Energy
R + X
RX
1 transition state
TS
R
Y
X
1 transition state
within a pre formed
aggregate
Reaction profile →
4
Y - -R - -X
RX + Y YR + X
1 transition state
Dissociative activation Associative activation
D I D I A A
YRX
RX + Y YR + X
2 transition states
and
1 intermediate
Figure 2.2: Potential energy profiles of different mechanisms occurring at a square
planar Pt(II) centre as proposed by Langford-Gray. 5,15
In a dissociative mechanism the activation energy is determined by the energy required
to break the bond of the leaving group whereas bond making is the main factor
determining the size of the activation energy for an associative mechanism.
2.2.1 The Dissociation Mechanism (D)
Although, dissociative mechanisms of square-planar complexes have been rare in the
past, Tobe and Burgess 15 reported that the dissociative mechanism change over is
favoured by; 5
A strong trans σ-donor ligand, i.e. the amount of negative charge that the ligand
donates to the metal ion through the σ-bond will enhance the weakening of the
metal-leaving group bond; thereby stabilizing the 14-electron transition state by
increasing in electron charge density at metal centre. This prevents the facile
approach of the entering nucleophile.
A sterically hindered metal centre that possesses high electron density at the
metal atom destabilizes the ground state of the complex by decreasing the
electrophilicity of the metal centre and as a result prevents attack of nucleophile
from the axial position, either from below or above the Pt(II) centre.
A strong coordinating solvent will readily displace the leaving group; thereby
enhancing the formation of the 14-electron coordinate intermediate.