The reaction of MCl2 (M = Pd, Pt) with one mole-equivalent of L[L = 2-(MeSCH2)C5H4N (L-1), 2-(PhSCH2)C5H4N (L-2), 2-(MeSeCH2)C5H4N (L-3)] in MeCN gave the monomeric complexes [MCl2L] in good yields. Single-crystal X-ray diffraction studies of [MCl2L2] confirmed the complexes to be monomeric with square-planar geometry about the metal centre. Variable -temperature NMR spectroscopy showed that the complexes undergo fluxional processes in solution, which were confirmed, by DFT calculations, to be (E)-pyramidal inversion at the chalcogen centre rather than ring-flip or (E)dissociation processes. The calculations showed that the barrier to inversion increases in the order S < Se < Te. All complexes were characterised using H-1 NMR, UV/Vis and IR spectroscopy and microanalysis. (c) Wiley-VCH Verlag GmbH T Co. KGaA, 69451 Weinheim, Germany, 2005.