The present cobalt ion containing complexes 1–6 were found to exhibit potent cytotoxic effect towards a panel of cancer and normal cells, including HeLa (cervical cancer), MCF-7 (breast cancer), H1299 (lung cancer), H1975 (EGFR mutant lung cancer), A549 (EGFR wild-type lung cancer), LLC-1 (mouse lung cancer), HepG2 (liver cancer) and Hep3B (liver cancer) cells, and normal human LO2 hepatocytes. The gene discussed is EGFR; the disease is breast cancer.