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Zirconium complexes of mendiaxon, warfarin, coumachlor, and niffcoumar have been synthesized by reaction of the ligands with zirconium chloride in stoichiometric ratio 1:2. The formation of the complexes has been proved on the basis of elemental analysis, IR‐spectroscopy, ¹H‐NMR spectroscopy, and thermal studies. Differential thermal analyses and thermogravimetric analyses have been applied to study the compositions of the new complexes. It is concluded that the lactone‐ and the keto‐carbonyl groups of warfarin, coumachlor, and niffcoumar are bonded to the metal ion as bidentate ligands, but mendiaxon is bonded as monodentate ligand. Cytotoxic screening by MTT‐assay was carried out. Among these compounds the zirconium complex of mendiaxon showed highest cytotoxic activity against human promyelocytic leukemic HL‐60 cells. The inorganic salt was found to be active against this cell line.