Ru-II sulfoxide-maltolato complexes, Ru(ma)(2)(L)(2) (L = DMSO (1a) and TMSO (1b) or L-2 = BESE (1c)), were synthesized, as well as the analogous ethylmaltolato derivatives, Ru(etma)(2)(L)(2) (2a-c) (ma = 3-hydroxy-2-methylpyran-4-onate, etma = 2-ethyl-3-hydroxypyran-4-onate, TMSO = tetramethylene sulfoxide, BESE = 1,2-bis(ethylsulfinyl)ethane). A Ru-II bidentate sulfoxide-metronidazole complex, RuCl(BESE)(metro)2 (3), was also synthesized (metro = metronidazole = 2-methyl-5-nitroimidazole-1-ethanol), The complexes were characterized generally by H-1 NMR, UV-vis, and IR spectroscopies, as well as MS, elemental analysis, solution conductivity, and cyclic voltammetry. The molecular structures of Ru(ma)(2)(S,R-BESE) (1c) and trans-RuCl2(R,R-BESE)(metro)(2) (3) were determined by X-ray crystallography. All sulfoxide ligands are S-bonded. The complexes were tested against human breast cancer cells (MDA-MB-435S) using an in vitro MTT assay, a colorimetric determination of cell viability: 2a,b exhibit the lowest IC50 values of 190 +/- 10 and 220 +/- 10 muM, respectively. Cisplatin exhibits an IC50 value of 30 +/- 5 muM.