The water-soluble Ru-III maltolato, ethylmaltolato, and pyridonato complexes, Ru(O-O')(3) (O-O' = ma (1a), etma (1b), pyd (2a)), were synthesized (Hma = 3-hydroxy-2-methyl-4-pyrone, Hetma = 2-ethyl-3-hydroxy-4-pyrone, Hpyd = 3-hydroxy-1,2-dimethyl-4-pyridone). The complexes were characterized by elemental analysis, NMR and IR spectroscopies, MS, solution conductivity, and cyclic voltammetry, and in the case of Ru(ma)(3), by X-ray crystallography, which revealed a mer configuration. The paramagnetic H-1 NMR resonances of 1a, 1b, and 2a were assigned using 2D methods (H-1 COSY and H-1-C-13 HMQC) and variable-temperature H-1 NMR data and showed that 1a and 1b exist in aqueous solution predominantly as a mer isomer, while 2a is a mixture of mer and fac isomers. Although a C-13 NMR spectrum could not be measured directly for la, a partial C-13 spectrum was generated from the H-1-C-13 HMQC spectrum. Complexes la and 1b were tested for anti-proliferatory activity against the human breast cancer cell line MDA-MB-435S and gave IC50 values of 140 and 90 mu M, respectively.