The coordination and organometallic chemistry of a series of diazene (N-2)-bridged Ni(II) catalysts derived from pyrazole-scaffold-based ligands bearing alkyl and aryl pendent arms was investigated. Binucleating ligands were obtained as products of the condensation reaction between 3,5-dichloroformyl-1H-pyrazole and aliphatic/aromatic primary/secondary amines under anhydrous conditions. The Ni(II) catalysts were activated with ethyl aluminum sesquichloride (EASC) to oligomerize the ethylene mainly into C4, C6, C8, and C10 fractions with activities up to 1.2 and 0.5 x 10(6) g (mol-Ni)(-1) bar(-1) h(-1) at 30 and 50 A degrees C, respectively. All catalysts were found to be electrochemically active in the working potential range of -2 to +2 V. A change in the potential of Ni(II) was provoked by the N-4 donor bridging ligands, increasing the ethylene oligomerization activity.