Trans complexes such as trans-[PtCl2(NH3)(2)] have historically been considered therapeutically inactive. The use of planar ligands such as pyridine greatly enhances the cytotoxicity of the trans geometry. The complexes trans-[PtCl(R 'R " SO)(A)(2)]NO3 (R 'R " SO = substituted sulfoxides such as dimethyl (Me2SO), methyl benzyl (MeBzSO), and methyl phenyl sulfoxide (MePhSO) and A = NH3, pyridine (py) and 4-methytpyridine or picoline (pic)) were prepared for comparison of the chemical reactivity between ammine and pyridine ligands. The X-ray crystal structure determination for trans-[PtCl(Me2SO)(py)(2)]NO3 confirmed the geometry with S-bound Me2SO. The crystals are orthorhombic, space group P2(1)2(1)2(1). With Cell dimensions a = 7.888(2) Angstrom, b = 14.740(3) Angstrom, 15.626(5) Angstrom, and Z = 4. The geometry around the platinum atom is square planar with l(Pt-Cl) = 2.304(4) Angstrom, l(Pt-S) = 2.218(5) Angstrom, and l(Pt-N) = 2.03(1) and 2.02(1) Angstrom. Bond angles are normal with Cl-Pt-S = 177.9(2)degrees, Cl-Pt-N-1 = 88.0(4)degrees, Cl-Pt-N-2 = 89.3(5)degrees, S-Pt-N-1 = 93,8(4)degrees, S-Pt-N-2 = 88.9(4)degrees, and N-1-Pt-N-2 = 177.2(6)degrees. The intensity data were collected with Mo Ka radiation with lambda = 0.710 69 Angstrom. Refinement was by full-matrix least-squares methods to a final R value of 3.80%. Unlike trans-[PtCl2(NH3)(2)], trans-[PtCl2(A)(2)] (A = py or pie) complexes do not react with Me2SO. The solvolytic products of cis-[PtCl2(A)(2)] (A = py or pie) were characterized. Studies of displacement of the sulfoxide by chloride were performed using HPLC. The sulfoxide was displaced faster for the pyridine complex relative to the ammine complex. Chemical studies comparing the reactivity of trans-[PtCl(R 'R " SO)(amine)(2)]NO3 with a model nucleotide, guanosine 5'-monophosphate (GMP), showed that the reaction gave two principal products: the species [Pt(R 'R " SO)(amine)(2)(N7-GMP)], which reacts with a second equivalent of GMP, forming [Pt(amine)(2)(N7-GMP)(2)]. The reaction pathways were different, however, for the pyridine complexes in comparison to the NH3 species, with sulfoxide displacement again being significantly faster for the pyridine case.