2-[1-Oxo-1-(piperidin-1-yl)propan-2-ylidene]-N-(prop-2-en-1-yl)hydrazinecarbothioamide (HL) and its six coordination compounds [Cu(L)X] (X = Cl⁻ (1), NO₃⁻ (2)), [Cu(A)(L)NO₃](A = 1,10-Phen (3), 2,2′-Bpy (4)), [Ni(HL)₂](NO₃)₂ (5), and [Fe(L)₂]Cl (6) are synthesized and characterized by elemental analysis, Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), and single-crystal X-ray crystallography. Uncoordinated thiosemicarbazone HL shows higher antioxidant activity against 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid (ABTS^+•) cation radicals compared with most of the other complexes. Complex 5 is the most active and its activity exceeds that of HL and is close to that of trolox, which is used in medicine as an antioxidant. The introduction of N-heteroaromatic bases (1,10-phenanthroline and 2,2′-bipyridine) into the inner sphere of the complex [Cu(L)NO₃] did not lead to an increase in antioxidant activity.