The synthesis of a 5-pyrazolone derivative (H2L) bearing a carbothioamide group at N1, N-ethyl-4-(2-hydroxyethyl)-3-methyl-5-oxo-2,5-dihydro-1H-pyrazole-1-carbothioamide, using a Keggin catalyst (H4SiW12O14·nH2O), is described. Complexes of copper(II) and nickel(II) have been prepared with this potential ligand and studied by potentiometry. Processing the pH-metric titrations revealed remarkable chelation ability of the ligand and high stability constants for the complexes. The complexes have been characterized by IR, UV/visible, electrochemistry, magnetic measurements, Electron Paramagnetic Resonance (EPR) and single-crystal X-ray diffraction. The structures are [Cu(HL)2H2O]·2H2O and [Ni(HL)2H2O]·2H2O, where the metal is five coordinate in an almost perfect trigonal bipyramidal geometry (D3h). The pyrazolone derivative (H2L) is a NS bidentate negative ligand, which coordinates to the metal through sulfur and amine nitrogen of the pyrazolone ring. The electronic results and the EPR of the complexes have been carefully studied in this (D3h) crystal field with a special emphasis on the energy levels. DNA-binding experiments have been conducted by UV spectroscopy and electrochemistry. The results reveal an interaction between the complexes and DNA and suggest an intercalative binding mode. UV titrations allowed determination of the binding constants Kb and revealed that the nickel(II) complex binds more strongly