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Keywords: Mononuclear Zn complexes ESI(+)-MS/MS Crystal structure Antibacterial activity Cytotoxic activity Transmission electron microscopy (TEM) a b s t r a c t The synthesis and characterization of the four zinc(II) complexes [Zn(HL 1)Cl 2 ] (1), [Zn(H 2 L 2)Cl 2 ](2), [Zn(H 2 L 3)Cl 2 ] (3) and[Zn(H 2 L 4)Cl 2 ] (4), where HL 1 = (bis-2-pyridylmethyl)amine, H 2 L 2 = (2-hydroxyben-zyl-2-pyridylmethyl)amine, H 2 L 3 = N-2[(pyridine-2-ylmethyl)amino)ethanol, H 2 L 4 = 1-[(pyridine-2-ylmethyl)-amino]-propan-2-ol are reported; (3) and (4) are new while (2) was reported previously but its structure had not been determined. The complexes were characterized by elemental analysis, IR, UV–Vis and NMR spectroscopic, electrospray ionization mass spectrometry (ESI(+)-MS) and tandem mass spectrometry ESI(+)-MS/MS). X-ray diffraction studies were performed for complexes (1)–(3) revealing the presence of mononuclear structures in the solid state. The X-ray analyses of (1) and (3) demonstrate that HL 1 and HL 2 act as tridentate ligands, while the ligand H 2 L 2 in (2) is bidentate. The cytotoxic properties of the ligands and of all the complexes were examined using human leukemia THP-1, U937 and Molt-4 cells. Complex (4) exhibited the highest cytotoxicity in this series with an IC 50 value of 75 ± 1 lmol L À1 against U937 cells. Transmission electron microscopy (TEM) reveals ultrastructural changes typical of apoptotic cells. The induction of apoptosis was confirmed by the annexin V assay. The antimicrobial activity of complexes (1)–(4) was also investigated in vitro against four Gram-positive bacteria (ATCC10832, ATCC25923, COL) and the clinical Staphylococcus aureus isolate LSA88 (SEC/SEF/ TSST-1+). Complex (2) showed the most potent inhibitory activity, reaching almost 100% of inhibition against all strains tested. Morphological investigations using TEM indicate that the antibacterial activity of complex (2) may be associated with the inhibition of cell wall and therefore cell division.