Tuning Reactivity of Platinum(II) Complexes

destabilisation. It is suggested that if the ground-state effects are dominant relative to
transition state, then an unusual acceleration in the rate of substitution occurs in order
to relief the repulsive forces associated with a crowded metal centre. However, studies
have shown that steric hindrance through introduction of methyl and ethyl substituents
on the tridentate R5dien chelate slows down the substitution rate of a series of
[Pd(R5dien)Cl] + complexes, spanning 5 orders of magnitude in comparison to the
unhindered systems, without having any effect on the substitution mechanism (Table
2.4).
Table 2.4: Rate constants and activation parameters for the substitution of coordinated
chloride by I¯ in [Pd(Rndien)Cl] + (n = 0, 3-5) in aqueous solution at 25 °C. 45
R5dien k1, s -1 ΔH ≠, kJ mol -1 ΔS ≠, J K -1mol -1 ΔV ≠, cm 3 mol -1
dien 44 43 -69 -10.0
1,4,7-Et3dien 10 41 -86 -10.8
1,1,7,7-Et4dien 2.2 x 10 -3 66 -74 -14.9
1,1,4,7,7-
me5dien
1,1,4,7,7-
Et5dien
0.28 x 10 -4 50 -88 -10.9
7.2 x 10 -4 59 -106 -12.8
It is known that the large negative values of activation entropies and negative volumes
of activation are indicative of associative mode of substitution mechanism. However, it
is difficult to make a distinction between true steric effects and electronic influence of
the alkyl substituents or a change in mechanism through the series. This is because at
80 °C [Pd(R5dien)Cl] + complex was found to follow the dissociative mechanism.
2.5.4 Effect of Solvent
The solvent is the reaction medium and through solvation of the ground and the
activated states it may influence the energy of activation process. The two-term rate
law 15 (Equation 2.13) for the substitution reactions of d 8 square-planar complexes
includes a term that is independent of the incoming nucleophile. This is ascribed to a
parallel solvolysis pathway that involves direct participation of the solvent through
33