Fe (III), Co (II), Ni (II), Cu (II), Y (III), Zr (IV), La (III), and U (VI) interact with Gat-o-phdn Schiff base (4E,40E)-4,40-(1,2-phenylenebis (azaneylylidene)) bis(1-cyclopropyl-6-fluoro-8-methoxy-7-(3-methylpiperazin-1-yl)-1,4-dihydroquinoline- 3-carboxylic acid to produce cationic mononuclear complexes. The isolated solid complexes were characterized with physicochemical, spectroscopic techniques (FT-IR, UV–Vis and 1H NMR), mass spectrometry, and thermogravimetric analyses. The infrared data indicated that Gat-o-phdn acting as tetradentate ligand chelated to the metal ions through the carboxylate oxygen and the nitrogen of azomethine group. The metal ions complete the coordination number with water molecules. The mechanism of the thermal decomposition was detected, and the kinetic parameters of the dissociation steps were evaluated using Coats–Redfern (CR) and Horowitz–Metzger (HM) methods. Bond lengths, bond angles, total energy, heat of formation, dipole moment, and the lowest energy model structures have been determined using density functional theory (DFT) calculations. The synthesized ligand and its complexes were screened for antimicrobial activities against two Gram-positive bacteria, two Gram-negative bacteria, and two fungi. The Cu (II) complex was very highly significant against Staphylococcus aureus compared with free Gato- phdn and references standard control. Also, it showed the highest antibacterial effects compared with all complexes.