Stability of methyl platinum complexes in water: The role of pH and geometry

Several water-soluble platinum(II) complexes containing methyl ligands have been prepared, PtMe₂L₂ and Pt(Cl)(Me)L₂ (L₂ = two P(m-C₆H₄SO₃Na)₃, TPPTS; (m-C₆H₄SO₃-Na)₂PCH₂ CH₂ P(m-C₆H₄SO₃Na₂), DPPETS; (m-C₆H₄SO₃Na)₂ PCH₂CH₂CH₂ P(m-C₆H₄SO₃Na)₂, DPPPTS; or (m-C₆H₄SO₃Na)₂PCH₂ CH₂CH₂ P(m-C₆H₄SO₃Na)₂, DPPBTS), to establish the stability of the platinum methyl bond in aqueous solution. From pH = 3 to 14 there is no reaction other than ligand substitution of H₂O or OH^- for Cl^-; there is no CH₃OH elimination. At lower pH's protonolysis of the Pt-Me bond occurs, producing CH₄. Dissolving PtMe₂L₂ into a solution of HCl at pH = 1 rapidly produces PtCl₂L₂ for the bidentate ligands, but trans-Pt(Cl)(Me)L₂ is observed for the monodentate ligands. trans-PtClMe(TPPTS)₂ undergoes protonolysis very slowly, indicating an important role for geometry in protonolysis reactions. The protonolysis reactions occur stereospecifically but are sometimes accompanied by cis-trans isomerization. The cis or trans thermodynamic preferences, cis-PtCl₂L₂, cis-PtMe₂L₂, cis-Pt(OH)(Me)L₂, trans-Pt(Cl)(Me)L₂, and trans-Pt(H₂O)(Me)L₂⁺, are not easily explained.