Aqua Group Acidity in Complexes of the Type trans-[Pt(NH3)2(L)(H2O)] (with L = Substituted Pyridines). Linear, yet Weak Dependence of pKa of the Aqua Ligand from L Basicity

The pKa of the aqua ligand in complexes of the type trans-[Pt(NH3)2(L)(H2O)] depends on the nature of the ligand trans to H2O, as expected. With L = NH3 or NH2R and n = 2, the pKa is around 6.0 – 6.4. With L = N-heterocyclic ligands generally higher acidities of the aqua ligands are observed, with pKa values being in the range of 4.7 – 5.4. These values are between those for L = H2O, n = 2 (4.4) and L = Cl−, n = 1 (5.7). For differently substituted pyridine ligands L a linear relationship exists between the pKa of the H2O ligand and the basicity of the heterocyclic ligand L, which is relatively weak, however.