<p>Two Schiff base ligands, namely, HL₁ and HL₂, were synthesized by condensing 3,5-diamino-1,2,4-triazole with 3-bromobenzaldehyde and 2,4-dichlorobenzaldehyde, respectively. Six mixed-ligand complexes were then synthesized using these Schiff base ligands with Co(II), Ni(II), and Cu(II) metals in the presence of 1,10-phenanthroline as a co-ligand. The compounds were structurally and physiochemically characterized using elemental analysis, conductivity measurements, FT-IR, UV–visible, ¹H and ¹³C NMR spectroscopy, mass spectrometry, magnetic susceptibility, powder XRD, and TGA. Spectroscopic data confirmed azomethine formation and coordination through nitrogen donor atoms of both ligands. Molar conductance indicated 1:1 electrolyte behavior for most complexes, except the Ni(II)-HL₂ complex. Magnetic and electronic spectral evidence showed octahedral geometry for Co(II) and Ni(II) complexes and distorted square-planar geometry for Cu (II) complexes. From the results of XRD, Co(II)-HL₂ and Ni(II)-HL₂ complexes gave nanocrystalline character, whereas the remaining complexes are amorphous solids. DFT calculations at the level of B3LYP confirmed such geometries and revealed that HL₁ is more chemically reactive, HL₂ being more stable. Biological testing of the free ligands showed no antibacterial activity, whereas the metal complexes exhibited moderate antibacterial activity against Escherichia coli and Staphylococcus aureus. The DPPH antioxidant test revealed that the Co(II)-HL₁ complex possessed the best free radical scavenging performance (with an inhibition of 22.51%). These indicate how metal ion interaction can dramatically modify structural, electronic, and biological properties of these ligands.</p> Graphical abstract <p></p>