An unusual 2:1 aggregate between the complex trans-[PtCl(CH3)(DMSO)(2)] (1; DMSO = dimethyl sulfoxide) and the organotin compound bis(mu(3)-oxo)bis(mu-chloro)bis(mu-dimethyltin(IV))bis(chlorodimethyltin(IV)) [Cl(CH3)(2)-SnOSn(CH3)(2)Cl](2) has been isolated and characterized by X-ray analysis. The crystals belong to the triclinic space group with lattic constants a = 9.373(2) Angstrom, b = 9.576(1) Angstrom, c = 14.087(3) Angstrom, alpha = 70.29(1), beta = 75.50(1)degrees, gamma = 72.21(2)degrees, and Z = 2. Least-squares refinement of the structure led to an R factor of 2.37%. H-1, C-13, and (195)PtNMR measurements revealed that in chloroform solution complex 1 gives a mixture of four different species, which have been unambigously identified as the starting complex 1 in equilbrium with cis-[PtCl(CH3)-(DMSO)(2)] and the two corresponding isomeric aqua-species cis and trans-[PtCl(CH3)(DMSO)(2)](OH2)]. The Pt-195 NMR magnetization transfer technique allowed determination of the rate of interconversion among the various complexes, showing that the direct trans-cis isomerism between the [PtCl(CH3)DMSO)(2)] species is negligible and that the geometrical interconversion occurs through a water-catalyzed pathway. The exchange between free and coordinated DMSO in cis- and trans-[PtCl(CH3)(DMSO)(2)] has been measured by H-1 NMR magnetization transfer experiments. The molecule of DMSO in the position trans to a methyl group was found to be 10-fold more labile than that trans to another sulfur bonded dimethyl sulfoxide. The reactivity of complex 1 in chloroform with a series of monodentate nitrogen ligands having widely different electronic and steric properties has been investigated by H-1 NMR spectorscopy. The utility of this system as precursor for the synthesis of cis and trans-[PtCl(CH3)(DMSO)(am)] is discussed together with the evidence for the factors promoting the prevalence of a geometrical configuration.