Under identical synthetic conditions, the reaction of aqua-nitro complexes of ruthenium nitrosyl with methyl-substituted pyridines results in drastically different products. While 3- and 4-MePy react similarly to pyridine giving monomeric complexes [RuNO(NO2)2L2OH] with good yields (70-80%), the sterically hindered 2-MePy forms a dimeric complex [(RuNO(2-MePy)(NO2)(μ-NO2))2μ-O] with only one N-heterocycle coordinated to the ruthenium center. Further investigation of photoinduced (445 nm) isomerization of Ru-NO (GS) to Ru-ON (MS1) at 80 K also shows different behaviors. Complexes with 3- and 4-MePy form metastable states with populations equal to 25% and 40% according to IR spectroscopy data. Activation parameters for the reverse decay reaction (MS1 → GS) determined from DSC are lgk0 = 12.0(0.2), Ea = 56.0(0.7) kJ mole-1 and lgk0 = 13.7(0.1), Ea = 61.8(0.4) kJ mole-1 for the 3-MePy and 4-MePy complex. A dimeric complex with 2-MePy does not undergo any changes after irradiation. Estimates of cytotoxicity in human mammary adenocarcinoma (MCF-7) and immortalized human embryonic kidney (HEK-293) cell lines demonstrate that complexes with 3-MePy and 4-MePy exhibited a 50% decrease in the growth of cancer cells at remarkably low and comparable concentrations (0.82-3.2 μM).