A series of pyrazolate-based dinuclear Ni(II) complexes relevant to the active site of urease are reported. Deprotonation of HL1 [HL1 = 3,5-bis(R2NCH2)-pyzH; R2N = Me2N(CH2)(3)NMe] by means of 1 equiv of BuLi and subsequent reaction with 2 equiv of [Ni(H2O)(6)](ClO4)(2) in the presence of (NEtPr2)-Pr-i affords the dinuclear complex [(LNi2)-Ni-1(OH)(MeCN)(2)](ClO4)(2) (1). This is shown crystallographically to contain two five-coordinate nickel ions bridged by both the pyrazolate and a hydroxide, with an acetonitrile solvent molecule bound to each metal center. When HL2 is employed {HL2 = 3,5-bis(R2NCH2)-pyzH; R2N = [Me2N(CH2)(3)](2)N}, the additional ligand side arms act as proton accepters forming an intramolecular N .. H .. N bridge to yield the complex [(HLNi2)-Ni-2(OH)-(MeCN)(2)](ClO4)(3) (2), whose basic bimetallic framework is essentially identical to 1. The two Ni(II) centers in 2 exhibit strong antiferromagnetic coupling (J = -46.7 cm(-1)). The labile acetonitrile donors in 2 are easily replaced by either neutral ligands such as dmf or anions such as thiocyanate, giving rise to the formation of complexes [(HLNi2)-Ni-2(OH)(dmf)(2)] (ClO4)(3) (3) and [(HLNi2)-Ni-2(OH)(NCS)(2)](ClO4) (4), respectively, where the overall dinuclear framework of 2 remains unchanged upon the substitution reaction.