Tuning Reactivity of Platinum(II) Complexes

the complex, where k2(Cl)[ 36Cl ¯] > k-2. As a result larger rates occur in nonpolar solvents
like tetrachloromethane, where the Cl ¯ would not be solvated compared to polar
solvents such as dimethylformamide (DMF).
2.5.5 Effect of Non-participating Groups
2.5.5.1 cis-and trans-Effects of Platinum(II) Complexes
The effect that a ligand has on another in the cis and trans positions is referred to as the
cis- and trans-effects, respectively. Chernayev and colleagues 48 introduced these studies
while they investigated square-planar platinum (II) complexes. Both the trans- and cis-
carrier ligands affect the rate of substitution at the metal centre, but in different ways. 15
The effect on the reactivity is more pronounced from non-labile ligands that are in
trans-position to the leaving group than it does on the ligand that is in the cis-
position. 19,26
2.5.2.2 The Trans-effect
The trans-effect is best defined as the effect of a coordinated ligand upon the rate of
substitution of ligands opposite to it; or is the ability of a ligand in a square planar
complex to direct the replacement of the ligand trans to it. 8,35 From extensive
investigations performed on the effect of the ligand on substitution reactivity at a Pt(II)
centre, using many different nucleophiles, the trans-effect in increasing order of power
is : 6,8,49
CO ~ C2H4 ~ CN ¯ > R3P ~ H ¯ > CH3 ¯ C6H5 ¯ > I ¯ >SR2 > SC(NH2)2 > SO3 2- > NO2 2- ~N3 - <
SCN - ~ Br ¯ > Cl ¯ > py > RNH2 ~ NH3 > OH ¯ >H2O.
This sequence covers the rate effects in the range 10 6 to 10 11-fold increase on the
lability of the complex when a strong trans-labilizing ligand is present. 50,51 Since the
trans-effect is a kinetic phenomenon, its origin lies in reactant destabilisation and /or in
the transition state stabilisation. In order to have a better insight of the trans-effect
series and acceleration of the rate of substitution, its effect at the ground state and the
transition state levels should be considered. For instance, a stabilisation of the
transition state may reduce the activation energy barrier or a destabilisation of the
ground state would increase the rate. A ligand high up in the trans-effect series may
35