Tuning Reactivity of Platinum(II) Complexes

process is accelerated by protonation of the freed terminal N atoms of the bridging
ligand, which prevents re-attachment to the metal centre. All the values of the activation
entropies are negative and confirm the associative character of the nucleophilic
substitution mechanism.
In Chapter 6, thermodynamic and kinetic properties of relatively new diaqua Pt(II)
complexes of the type [{cis–Pt(NH3)2(H2O)}2L] +4 (L = 4,4`-bis(pyridine)sulphide, 4,4’-
bis(pyridine)disulphide or 1,2-bis(4-pyridyl)ethane ) were studied for the first time. The
study mainly focused on the influence of the spacer group and its effect on nucleophilic
substitution reactions. The results indicate that there is electronic communication
between the two platinum centres leading to two pKa values. The thermodynamic data
obtained demonstrates that the pKa values of the coordinated aqua moieties are
dependent on the nature of the linker. Clearly, higher pKa values were obtained as the Pt-
--Pt separation distance becomes longer showing that the effectiveness of localised
charge additions at the Pt(II) centres decreases as the distance is increased further. The
pKa2 values of the second deprotonation step are also influenced by nature of spacer
group, whereby S-based spacers are stronger σ-donors which impacts negatively on the
acidity of the second coordinated water molecule. This resulted in bigger pKa2 values as
the number of S atoms was increased.
It is also demonstrated by these investigations that the nature of the bridge has a strong
influence on the reactivity of the complexes. The results of the substitution reactions
with anions and thiourea nucleophiles of the corresponding diaqua complexes
performed at pH 2 were explained in terms of the adopted symmetry by the different
dinuclear complexes. The repulsions between the two lone pairs on S atom and the
pyridine rings prevent Pt1 to adopt a perfect C2v symmetry. The DFT calculations clearly
demonstrated that due to non-planarity of the ligand the structure of the Pt1 complex is
distorted. The resultant distortion imposes aerial steric hindrance at the Pt(II)) centre
Pt1 which is the reason why the reactivity of Pt1 is about 4–7 times lower than that of
the other complexes Pt2 and Pt3. The magnitude of the steric hindrance imposed by the
pyridyl bridge cis to the leaving group becomes weaker as the Pt---Pt separation
distance increases, leading to the increase in rates of substitution from Pt1 to Pt3 and
4