A series of new copper(I) and mercury(II) complexes with 4-aminoantipyrine, semicarbazide, and thiosemicarbazide ligands [(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)carbonohydrazonoyl dicyanide (HL1), 2-cyano-N-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)-2-[(E)-(3-methylphenyl)-diazenyl]acetamide (H2L2), (Z)-2-cyano-N′-[(E)-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)-methylidene]-2-(2-phenylhydrazinylidene)acetohydrazide (HL3), 2-(anilinoacetyl)-N-phenylhydrazine-1-carbothioamide (H2L4), 2-(anilinoacetyl)-N-(3-methylphenyl)hydrazine-1-carbothioamide (H2L5), and 2-anilino-N′-[(E)-(2-hydroxyphenyl)methylidene]acetohydrazide (H2L6)] have been prepared and characterized by physical and spectral data, including microanalysis, IR and UV-visible spectra, conductivity measurements, and thermal analyses (DTG/TGA). The ligands H2L2 and HL3 produced dinuclear complexes. Thermal studies revealed that the copper(I) complexes are thermally more stable than mercury(II) complexes. The copper complexes exhibited potent inhibitory effect on the MCF7 human breast carcinoma cell line, as compared to mercury(II) complexes.