Five new nickel (II) complexes, namely, [Ni(L1)2](ClO4)2(1); [Ni(L2)2](ClO4)2(2); [Ni(L3)2](ClO4)2(3); [Ni(L4)2](ClO4)2(4); [Ni(L5)2](ClO4)2(5), where L1 = benzoylhydrazide; L2 = N-[(1)-1-(2-methylphenyl)ethylidene]benzohydrazide; L3=N-[(1)-1-(4-methylphenyl)ethylidene]benzohydrazide; L4=N-[(1)-1-(2-methoxyphenyl)ethylidene]benzohydrazide; L5 = N-[(1)-1-(4-methoxy-phenyl)ethylidene]benzohydrazide, have been synthesized and characterized by various physicochemical and spectroscopic techniques. The synthesized complexes are stable powders, insoluble in common organic solvents such as ethanol, benzene, carbon tetrachloride, chloroform, and diethyl ether, and are nonelectrolytes. The magnetic and spectroscopic data indicate a distorted square planar geometry for all complexes. The superoxide dismutase activity of these complexes has been measured and discussed. Antibacterial and antifungal properties of these complexes were also tested. 1. Introduction The synthesis of low molecular weight nickel (II) complexes mimicking superoxide dismutase (SOD) activity has been challenging for bioinorganic chemists and recently some complexes with high catalytic activity have been reported [1–3]. Nickel-containing superoxide dismutase (Ni-SOD) has been isolated from several Streptomyces species [4]. The enzymatic activity of Ni-SOD [5] is as high as that of Cu-Zn SOD at about 109？M？1S？1 per metal center. Oberley and Buettner [6] have reported that cancer cells had less superoxide dismutase (SOD) activity than normal cells. Superoxide ion is toxic to cells; a defense mechanism must have been initiated by nature. All organisms, which use dioxygen and many that have to survive an oxygenated environment, contain at least one SOD. The superoxide radical ( ) is an inevitable byproduct of aerobic metabolism which if not eliminated may cause significant cellular damage and has been implicated in numerous medical disorders [7]. To avoid such harmful consequences, all oxygen metabolizing organisms possess metalloenzymes known as superoxide dismutases (SODs). These SODs disproportionate the toxic radical to molecular oxygen and hydrogen peroxide [8, 9]. All SODs employ the two-step Ping-Pong mechanism shown in where M is a redox active metal center capable of both oxidizing and reducing superoxide. Metal complexes of bidentate Schiff bases have been extensively studied [10, 11], because such ligands can bind with one, two, or more metal centers involving various coordination modes and allow synthesis of homo- and heteronuclear metal complexes with interesting stereochemistry [12,
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