The dinuclear complex [Co-2(mu-OH)(mu-XDK)(bpy)(2)(EtOH)](NO3), where XDK is the dinucleating dicarboxylate Ligand m-xylylenediamine bis(Kemp's triacid imide) and bpy = 2, 2'-bipyridine, was prepared as a functional model for arginase. The substrate aminoguanidinium nitrate was hydrolyzed to urea in ethanol by the complex but not by free hydroxide ion under the same conditions. The amino group of the substrate binds to cobalt, as demonstrated by W-vis spectroscopic studies. The syntheses of related dinuclear cobalt(II) complexes [Co-2(mu-XDK)(NO3)(2)(CH3OH)(2)(H2O)], [Co-2(mu-Cl)(mu-XDK)(bpy)(2)(EtOH)(2)](NO3), and [Co-2(mu-XDK)(py)(3)(NO3)(2)] are described. Mononuclear complexes [Co(XDK)(bpy)(H2O)] and [Zn(XDK)(bpy)(H2O)] were also prepared and characterized. The former catalytically hydrolyzes aminoguanidinium nitrate to urea in basic 1:1 methanol/water solutions, whereas the latter does not promote this reaction. Hydrolysis of aminoguanidinium ion is effected by [Co(CH3COO)(2)] and [Cu(CH3COO)(2)] in the presence of bpy, but not by [Zn(CH3COO)(2)], [Ni(CH3COO)(2)], or [Mn(CH3COO)(2)] in the presence of bpy in 1:1 methanol/water solution. In all cases, coordination of the amino group of the substrate to the metal center under the reaction conditions may activate the leaving group and orient the guanidinium moiety close to the attacking nucleophile, metal-bound hydroxide ion, to promote the hydrolysis reaction.