New Co(II), Ni(II), Cu(II), Zn(II), and Fe(III) complexes of the ligand 3-(1-methyl-4-hydroxy-2-oxo-1,2-dihydroquinolin-3-yl)-2-nitro-3-oxopropanoic acid (H 3 L) were synthesized and characterized by various analytical and spectroscopic techniques. The obtained data confirmed the formation of 1:1 (M:L) stoichiometry for Fe(III), Co(II) complexes and 2:1 (M:L) stoichiometry for Ni(II), Cu(II), Zn(II) complexes, and the ligand, H 3 L, acts as mono-, bi-, tribasic bis-, tris-and tetradentate towards the metal ions via the oxygen of quinolinonate, the enolic/ketonic of β-ketoacid, and carboxylate besides of the nitro groups. All complexes showed tetrahedral geometry, except Fe(III) complex which exhibited an octahedral geometry. The X-ray diffractograms of the ligand, H 3 L, and its metal complexes revealed a series of sharp and intense diffraction peaks which indicate their crystalline characteristic with the particle size in the nano-scale range. Geometrical optimizations of the ligand, H 3 L, and its metal complexes were performed at the B3LYP/6-311G(d, p) level using density functional theory (DFT) implemented in the Gaussian 09 program.The data obtained were correlated with the experimental results and suggested that the Zn(II) complex may exhibit extremely bio-efficiency than other compounds. The antimicrobial screening results showed enhancement in the efficiency of the free ligand, H 3 L, on coordination with the metal ions. The antitumor activity was evaluated in vitro against Hepatocellular carcinoma cell line (HepG-2 cells). The ligand, H 3 L, and its Zn(II) complex showed strong antitumor activities, especially nano-sized Zn(II) complex that exhibited IC 50 value (2.90 μg/ml) smaller than the well-known standard antitumor drug cisplatin (IC 50~3 .27 μg/ml), prominent it as an antitumor agent towards HepG-2 cells.