| Summary: | Two zinc(II) complexes with dimethyl 2,2′-bipyridine-4,5-dicarboxylate (py-2py) of the general formula [Zn(py-2py)X<sub>2</sub>], X = Cl<sup>−</sup> (<b>1</b>) and Br<sup>−</sup> (<b>2</b>) were synthesized and characterized by NMR, IR and UV-Vis spectroscopy and single-crystal X-ray diffraction analysis. Complexes <b>1</b> and <b>2</b> are isostructural and adopt a slightly distorted tetrahedral geometry with values of tetrahedral indices τ<sub>4</sub> and τ’<sub>4</sub> in the range of 0.80–0.85. The complexes were evaluated for their in vitro antimicrobial activity against two bacterial (<i>Pseudomonas aeruginosa</i> and <i>Staphylococcus aureus</i>) and two fungal strains (<i>Candida albicans</i> and <i>Candida parapsilosis</i>), while their cytotoxicity was tested on the normal human lung fibroblast cell line (MRC-5) and the model organism <i>Caenorhabditis elegans</i>. Complex <b>1</b> showed moderate activity against both <i>Candida</i> strains. However, this complex was twofold more cytotoxic compared to complex <b>2</b>. The complexes tested had no effect on the survival rate of <i>C. elegans.</i> Complex <b>2</b> showed the ability to inhibit filamentation of <i>C. albicans</i>, while complex <b>1</b> was more effective than complex <b>2</b> in inhibiting biofilm formation. The interactions of complexes <b>1</b> and <b>2</b> with calf thymus DNA (ct-DNA) and bovine serum albumin (BSA) were studied to evaluate their binding affinity toward these biomolecules.
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