Reactions of fac-[PtMe2(OMe)(H2O)3]+ with halide ions: effect of halide trans effect on methoxide hydrolysis

A solution of fac-[PtMe2(OMe)(H2O)3]+ (1) in aqueous perchloric acid underwent very slow hydrolysis of the PtOMe bond, over many weeks. When chloride was added to a solution of 1, two interconverting isomers of [PtMe2(OMe)Cl(H2O)2] (with chloride trans to methyl) were formed, and with excess chloride, [PtMe2(OMe)Cl2(H2O)]− (both chloride ligands trans to methyl). This solution was stable at ambient temperature, but on heating, methanol was formed and [PtMe2Cl2(H2O)2] (both chloride ligands cis to methyl) was produced in the solution. It is proposed that this reaction proceeds via an intermediate complex with chloride bound trans to methoxide. Concentration gave solid [{PtMe2Cl2}n], whose identity was confirmed by conversion to [PtMe2Cl2py2] (pyridine, py, trans to methyl). With bromide and iodide, methoxide hydrolysis occurred at ambient temperature, more slowly with bromide than with iodide, to form solid [{PtMe2X2}n] without significant concentrations of [PtMe2X2(H2O)2] formed as an intermediate. The greater tendency for PtOMe bond to hydrolyse trans to halide compared with 1 was ascribed to the higher trans effect of the halide ligand compared with that of water.