The work reported deals with the preparation characterization and studies of a number of new complexes of cobalt (II). with three ligands type of Schiff base dihydrazide the ligands are: dibenzoinlidenemalonoyldihydrazide (L 1 H 4 ), dibenzoinlidenesuccinyldihydrazide (L 2 H 4 ) and dibenzoinlidenephthalolyldihydrazide (L 3 H 4 ). The prepared complexes have been characterized the different studies by molar conductance, electronic spectra, and magnetic susceptibility ‫احمد‬ ‫علي‬ ‫صهباء‬

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... ‬ 298 measurements and infrared spectra. The different studies suggest that in neutral medium the ligands formed cationic complexes of the type [M(LH 4 )]X 2 . While in a basic medium anionic complexes of the general formula K 2 [M(L)] were obtained the complexes formed are also hexacoordinated with octahedral structures. Further study was conducted to have an idea about the biological activity of the ligands and their complexes against species of gram positive: Staphylococcus aureus It was found that these complexes have certain degree of reactivity, which they are more active in comparison to their ligands. Also the Minimum Inhibitory Concentration (MIC) was determined for some complexes. In addition to that, the effect of these compounds at the molecular level was studied by measuring the changes of total protein for Staphylococcus aureus bacteria.