Reaction of the dinucleating ligand H3L (2-(2'-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-i midazolidine) with Ni(NO3)(2)center dot 6H(2)O produces the dimer of monomers [Ni(HL1)](2)(NO3)(2)center dot 4H(2)O center dot 4H(2)O) following the hydrolysis of H3L. If the reaction occurs in the presence of 2-formylphenol (Hfp) or 2,6-diformyl-4-methylphenol (Hdfp), this hydrolysis is prevented by incorporation of these co-ligands into the structure and stabilization of the new complexes [Ni2L(fp)(H2O)]center dot 3H(2)O center dot 3H(2)O) and [Ni2L(dfp)]center dot 4.5H2O center dot 4.5H(2)O), respectively. Complexes and may be considered to be structural models of the active site of urease, where coordination of the carbonyl ligand mimics binding of urea. In complex , coordination of terminal water reproduces the binding of this substrate of the enzyme to the active site. In both dinuclear complexes, the Ni-II ions are coupled ferromagnetically to yield S = 2 ground states, whereas complex exhibits weak intradimer antiferromagnetic exchange through hydrogen bonds. The magnetic data can be modeled by using the Van Vleck equation, incorporating intermolecular interactions, or by diagonalization of a spin Hamiltonian that includes single-ion anisotropy.