Tuning Reactivity of Platinum(II) Complexes

k obs(3rd) / s -1
2.40x10 -4
2.00x10 -4
1.60x10 -4
1.20x10 -4
8.00x10 -5
4.00x10 -5
0.00
TU
DMTU
TMTU
0.000 0.002 0.004 0.006 0.008 0.010
[NU]/mol dm -3
Figure 5.8: Concentration dependence of kobs(3rd) for the displacement of the bridging
ligand in pmn by thiourea and its substituted derivatives, pH = 2.0, T =
298.15 K, I = 0.10 M (0.01 M HClO4, adjusted with NaClO4).
The first and second substitution steps, k2(1 st ) and k2(2 nd ) respectively, represents the
consecutive substitution of the water ligand on the coordination sphere of each of the
Pt(II) centres of the dinuclear diaqua Pt(II) complexes. The second substitution step is
up to 2-orders of magnitude slower compared to the first one. This is ascribed to a
communication between the two metal centres after the first substitution which results
into a change in the environmental conditions around the second metal centre. 47 The
DFT calculations show a change in NBO charges if TU is substituted to one platinum
centre, i.e. from Pt(OH2) to Pt(TU), as can be seen in Table 5.1, suggesting that electronic
charge is conveyed through the bridging ligand. The third step involves the dissociation
of the bridging ligand from the Pt(II) centre. The degradation process occurs as a result
of the ability of the S-donor ligands to displace simultaneously both the linker and the
am(m)ine ligands through the trans-effect, a process that is accelerated by protonation
of the released nitrogen atom at pH 2.0 16,18 preventing any possibility of reverse
reaction and therefore, forces the reaction forward as shown in Scheme 5.4. The excess
25