Influence of PtII and PdII coordination on the equilibrium of 2,2′-dipyridylketone (dpk) with its hydrated gem-diol form (dpk·H2O)

2,2′-Dipyridylketone (dpk), when acting as a chelating ligand for PdII or PtII, is in slow equilibrium with its corresponding gem-diol form (dpk·H2O). In D2O, equilibrium constants K = (dpk·H2O)/(dpk) change from ca. 0.04 for the free ligand to ca. 3 in the corresponding complexes with cis-[Pt(H2O)2]2+. In solution, species of both ligands can be identified and differentiated by 1H NMR spectroscopy, and in the trinuclear μ-OH bridged PtII complex [Pt3(μ-OH)3(dpk·H2O)2(dpk)](NO3)3·4.5H2O (4), both types of ligands are present simultaneously in a ratio of (dpk·H2O):(dpk) = 2. As demonstrated with a series of PdII complexes containing dpk·H2O and dpk ligands, a straightforward differentiation is possible when DMSO-d6 is used as solvent, because then also the OH protons of dpk·H2O are observable. It is also shown that monocrystalline [PdCl2(dpk·H2O)] (1), when dissolved in DMSO-d6, partially converts, with loss of H2O, to [PdCl2(dpk)].