A series of nickel(II) complexes bearing phenyl ether-pyrazol ligands [NiCl2L] (Ni1: L=3,5-dimethyl-1-(3-phenoxypropyl)-1H-pyrazole; Ni2: L=1-(3-phenoxypropyl)-3-phenyl-1H-pyrazole; Ni3: L=3,5-di-tert-butyl-1-(3-phenoxypropyl)-1H-pyrazole; Ni4: L=1-(3-phenoxypropyl)-1H-pyrazole) were synthesized and fully characterized by elemental analysis, high-resolution mass spectrometry (HRMS) and X-ray photoelectron spectroscopy (XPS). X-ray photoelectron data illustrates that the probability of (co)existing dimeric species increases in the following order: Ni4 < Ni1 < Ni2 < Ni3. All nickel precatalysts, activated with methylaluminoxane (MAO), exhibited moderate to high activities for ethylene oligomerization [TOF = 18.4-45.7 x 10(3) mol(ethylene)(mol(Ni))(-1) h(-1))] with good selectivities for 1-butene produced (62.6-80.7%). The ligand environment regarding the substituents on the pyrazolyl unit as well as the reaction parameters influence the catalytic performance and selectivity toward production of 1-butene. When activated with ethylaluminum sesquichloride (Et3Al2Cl3, EASC), Nil displayed low catalytic activity (TOF = 9300 (mol C2H4).(mol Ni-1 h(-1)); however, the 1-butene selectivity was increased, attaining 92.5%. The use of triphenylphosphine (PPh3) as auxiliary ligand afforded highly active catalyst system [TOF = 118.3 x 10(3) mol(ethylene)(mol(Ni))(-1) h(-1))] with poor selectivity for production of 1-butene (13.7%). (C) 2013 Elsevier B.V. All rights reserved.