Flavonoids are natural products which are known to have biological activity for human health. In this study, new mixed ligand complexes of Ni(II) and Cu(II) were synthesized by using flavonoid (quercetin or naringenin) and heterocyclic imine (2,2':6',2 ''-terpyridine or 2,2'-bipyiridine) ligands. The new complexes are [Ni(narH-1)(terpy)Cl]center dot 4H(2)O (1, nar = naringenin, terpy = 2,2':6',2 ''-terpyridine), [Cu(narH-1)(terpy)Cl]center dot H2O (2), and [Cu(queH-1)(bpy)(O3N)]center dot 1.5H(2)O (3, que = quercetin, bpy = 2,2'-bipyiridine). The structural features of the synthesized mixed ligand complexes were investigated using elemental analysis, thermogravimetric analysis, Fourier transform infrared spectroscopy, magnetic susceptibility and molar conductivity measurements. The resulting data demonstrated an octahedral geometry for Complex 1 and Complex 2 and square pyramidal geometry for Complex 3. Antioxidant capacity and total phenolic content of Complexes 1-3 were measured by the Folin-Ciocalteu and ABTS methods. Anti- proliferative effect of complexes were tested by SRB and ATP assays on MCF-7 (breast cancer), A549 (nonsmall cell lung cancer), PC-3 (prostate cancer) and HeLa (human cervical cancer) cell lines. Apoptosis was identified using by the fluorescence imaging, caspase cleaved cytokeratin-18 and flow cytometry analysis. Complex 2 and 3 had high total phenolic content and antioxidant activity. Complex 2 was found to show selective cytotoxicity through the induction of apoptosis on MCF-7 cells with having a very low IC50 value (<0.8 mu M; the half maximum inhibitory concentration) while its ligands showed much higher cytotoxicity (IC50 > 50 mu M). In conclusion, Complex 2 is a highly promising and novel compound for breast cancer and warrants further animal experiments. (C) 2019 Elsevier B.V. All rights reserved.