Two tetrahedral mononuclear complexes with a general formula [CoX2(dmphen)](1–2) (where dmphen is 2,9-dimethyl-1,10-phenanthroline and 1 (X = Br), 2 (X = NCS)) have been synthesized. These complexes are characterized by elemental analysis, IR, UV–visible, TG/DTA and by X-ray diffraction. The calculated electrostatic potential surface of 2 has shown that the electrostatic potential values around sulfur atom is anistropically distributed; the potential values along C–S bond is less negative than the corresponding values in the π-region of S atom. This agrees with the observed geometrical arrangement of C–H⋯S–C hydrogen bonding interactions, theAVG. of H⋯S–C angle is 81°. Antimicrobial properties of cobalt(II) complexes was also assessed. Cobalt complexes exhibited significant antibacterial activity against different gram negative and positive human pathogens. The absorption spectrum of these complexes in acetone was modeled by time-dependent density functional theory (TD-DFT).