© 2020Novel mononuclear heteroligand transition metal complexes: [M(HL)(phen)2]ClO4, (M: Mn(II) for 1, Ni(II) for 2), [M(HL)(phen)(ClO4)], (Ni(II) for 3, Cu(II) for 4, Zn(II) for 5) with 2-[(E)-(hydroxyimino)methyl]-4-[(E)-phenyldiazenyl]phenol, H2L as primary ligand and 1,10- phenanthroline as bidentate co-ligand(s) in different mole ratios have been synthesized and characterized by using elemental analysis, FTIR, UV–Vis, NMR, MALDI-TOF mass spectrometry and thermal analysis. The complexes (1, 2) have distorted octahedral geometry while the complexes (3–5) have distorted square-pyramidal coordination geometry. In the complexes, the metal ion is coordinated to the deprotonated azo-oxime ligand through the phenolic oxygen atom and nitrogen of the imine. 1,10-Phenanthroline is coordinated to the metal ion through its two N-donors. The thermograms of all the complexes were confirmed the proposed structures. Time-dependent (TD) DFT-based calculations have been also performed for geometric optimization and to assign the experimental vibrational and electronic transition of the complexes. The superoxide-scavenging activities of the complexes were also investigated and IC50 values were evaluated. Among the complexes studied, the Cu(II) complex (4) exhibits the most activity with the lowest IC50 value (2.02 ± 0.15).