Preferential formation of cis- or trans-palladium(II) complexes controlled via the exocyclic binding sites embedded in dithiamacrocycles (L-1 = -S(CH2)(2)S-; L-2 = -S(CH2)(2)O(CH2)(2)S-) is reported. From the reaction with K2PdCl4, the shorter sulfur-to-sulfur separation in L-1 preferentially leads to the formation of cis-[Pd(L-1)Cl-2] (1), while L-2, incorporating a larger sulfur-to-sulfur separation, coordinates in a trans fashion to form a cyclic dimer, trans-[Pd-2(L-2)(2)Cl-4] (2). The observed results illustrate the possibility for the controlled formation of cis/trans square-planar complexes through binding-site design. When palladium(II) acetate was substituted for K2PdCl4 in the above reaction, L-1 gave no product, while L-2 resulted in the formation of a unique tripalladium(II) complex, [Pd-3(L-2)(CH3C(=N)OC(=N)CH3)(CH3COO)(4)] (3), in which three Pd-II atoms are linked by acetimidic anhydride, CH3C(=N)OC(=N)CH3, derived from the acetonitrile solvent employed. In the H-1 NMR spectrum for 3, specific methylene signals for methylene protons adjacent to S donors exhibit large complexation-induced splitting of the geminal proton signals into axial and equatorial proton peaks, thus indicating magnetically nonequivalent geminal protons that reflect the restricted conformation of the metallabicycle.