Denticity plays a prominent role in the cytotoxicity of metal complexes. The impact of ligand denticity of chromone/chromene thiosemicarbazones (TSCs) in the Ru(II)-DMSO complexes on the cytotoxicity of cancer cells was evaluated. Ru(II)-DMSO complex containing tridentate chromone TSC (D1) exhibited better in vitro anticancer activity than the one with bidentate chromene TSC (D2). Using the optimized structures of the complexes, the reason behind the same was attributed to the thermodynamically stable nature of the tridentate system. Hydrolysis study of the complexes showed the kinetically labile nature of the chloride ligand in the tridentate species with respect to the bidentate counterpart. The selective cytotoxicity of complex D1 against breast cancer cells and its less toxicity against normal cells demonstrated the effective anticancer potential of the complex. Further, the binding efficacy of the complexes against DNA and BSA, along with imaging and flow cytometry experiments, warranted the effective cell death induced by the complexes against breast cancer cells via apoptosis.