Complex-formation reactions of the L-cysteine-derived ligands N-alpha-acetyl-S-methylene-(2'-pyridine)-L-cysteine (py-CH2-accys) and N-alpha-acetyl-S-ethylene-2-(2'-pyridine)-L-cysteine (py-C2H4-accys) with [Pt(en)(H2O)(2)](2+) and [Pd(en)(H2O)(2)](2+) were investigated structurally by NMR spectroscopy and kinetically by UV-vis and stopped-flow spectrophotometry. The complexes [Pt(en)(py-CH2-accys-S,N)] (NO3)(2) (1) and [Pt(en) (py-C2H4-accys-S,N)] (NO3)(2) (2) were isolated and purified by reversed-phase HPLC. NMR spectroscopy revealed an S-thioether, N-pyridyl chelation mode with five- and six-membered chelate rings for 1 and 2 and their Pd(II) analogues 3 and 4. The amino acid functional group is not involved in coordination. Comparison of the second-order rate constants for the complex-formation reactions resulted in k(Pd(II))/k(Pt(II)) ratios of 3.7 x 10(4) for py-CH2-accys and 2.4 x 10(4) for py-C2H4-accys. In the presence of excess ligand, the trans effect of the Pd-coordinated sulfur donor induced labilization of the coordinated en ligand with successive displacement to give [Pd(py-CH2-accys)(2)] and [Pd(py-C2H4-accys)(2)]. The displacement reactions were shown to be in accordance with a preequilibrium behavior consisting of reversible formation of the ring-opened intermediate followed by successive irreversible ring closure. The results are discussed in reference to available literature data for related systems.