New mononuclear ternary transition metal complexes: [M(HL)(bipy)2]ClO4, (M: Mn(II) for 1, Ni(II) for 2), [M(HL) (bipy) (ClO4)], (M: Ni(II) for 3, Cu(II) for 4, Zn(II) for 5) with M(II), 2-[(hydroxyimino)methyl]-4-[-phenyldiazenyl] phenol, H2L, and 2,2'-bipyridine were synthesized, and their structures were investigated by using various analytical, spectroscopic techniques such as elemental analysis, FTIR, UV-Vis, NMR, MALDI-TOF mass spectrometry, thermal analysis. The theoretical studies were performed by DFT techniques by using B3LYP function with 6-311++G (d, p)/LanLD2Z basis set. The electronic transitions charters of the complexes were further analyzed by TD-DFT/CAM-B3LYP method. IR and thermal analysis data verify the proposed structures. The inhibition activities of the complexes against acetylcholinesterase (AChE) extracted from Ricania simulans adults and nymphs were examined and all the complexes were found to be active. Among the complexes studied, the highest inhibition activity was exhibited by complex 5 with the lowest IC50 value (3.2 +/- 0.8 mu M) for AChE of adults and complex 3 with the lowest IC50 value (4.6 +/- 0.8 mu M) for AChE of nymphs.