In the present contribution, a novel binuclear Ni(II) complex was synthesized from hexadentate imine ligand (NNO)2 which prepared from condensation of 9-hexyl-9H-carbazole-3,6-dicarbaldehyde and 2-amino-3,5-dibromobenzaldehyde and 5-bromo-2-hydroxyaniline using the ultrasonic method. The symmetric Schiff base ligand H2L= (C46H35Br6N2O2) and its nickel complex Ni2LCl2 were elucidated by different physicochemical methods including analytical analysis, magnetic moment measurement, FT-IR, 1H and 13C NMR, Mass, UV/Vis spectroscopy, TGA and field emission scanning electron spectroscopy (FE-SEM). Nano scale particles of nickel oxide were produced from the titled complex via the thermal decomposition method and analyzed by FT-IR, FE-SEM and X-ray powder diffraction (XRD) methods which indicated close accordance to standard pattern of NiO nano particles and showed acceptable size in nano range (32 nm). Density Functional Theory calculations were carried out at the B3LYP levels of theory with a double basis set LANL2DZ for nickel and 6-31 G (d,p) basis set for other atoms. The optimized geometry of the ligand and its nickel complex were obtained base on the optimized structures, the calculation of the FT-IR vibrational frequencies, 1H NMR and 13C NMR chemical shifts of the compounds were carried out and compared with those obtained experimentally. The results of this comparison demonstrated good agreement between theoretical and experimental data which can be a strong evidence to accuracy of experimental analytic data and validity of the applied mathematical model. In vitro antimicrobial activities of the Schiff base ligand and its nickel complex were investigated against some Gram-positive (Staphylococcus aureus and Eterococcus faecalis) and Gram-negative bacteria (Pseudomonas aeruginosa and Escherichia coli) and compared with each other. It was found that nickel complex showed higher activity.