Tuning Reactivity of Platinum(II) Complexes

eaction with different nucleophiles a small intercept corresponds to a large S
values. 18,26
Another important factor that affects the rate of substitution on the square-planar
geometry is the polarisability of the incoming nucleophile. In many substitution
reactions high nucleophilicity constants are reported for polarisable ligands such as
iodide and S-donor nucleophiles. 39 These are attributed to the large size and markedly
diffuse valence orbitals of the Pt atom, which enables the metal ion to form stronger
bonds with softer (easily polarisable) ligands. Thus, highly polarisable ligands
substitute the leaving groups more rapidly at the Pt(II) centre.
It is now clear that the n o pt scale is more applicable to charged nucleophiles. This
because charged nucleophiles have been observed to have a greater influence on
reactivity as opposed to steric crowding which only has a minor effect. 17,40 In addition
nucleophiles such as SeCN ¯ and NO2 2- deviate from LFER as seen in Figure 2.9. The
plausible explanation put forward is that such entering groups with better π-acceptor
ability would accelerate reactivity by lowering the energy of the transition state through
metal to ligand π-interactions. 38 Hence the complex [PtCl4] 2- would be more reactive to
these π-acceptor nucleophiles than [Pt(dien)Br] + that has a poor π-donating ability
relative to the reference trans-[Pt(py)2Cl2]. 41
2.5.2 Effect of the Leaving Group
The effect of the leaving group is of less importance in reactions following the
associative mode of substitution. However, in order for one to investigate the effect of
the leaving group the cis and the trans ligands should be the same.
The reaction below has been extensively studied. 42,43
[Pt(dien)X] (2-n) + py [Pt(dien)py] 2+
The data for this reaction is shown in Table 2.2.
28
+ X n-
(2.44)