Monometallic iron(II) and cobalt(II) complexes derived from potential hemilabile heterocyclic (pyrazolyl)imine catalyze ethylene oligomerization reactions upon activation with either EtAlCl₂ or MAO to give predominantly butenes and hexenes. The catalytic activities are largely controlled by the complex structure and stabilization from the coordinated ligand.

Reactions of heterocyclic (pyrazolyl)imine ligands (L1-L4) with CoCl₂ or FeCl₂ yield the complexes Co(L1)Cl₂ (Co1), Fe(L1)Cl₂ (Fe1), Co(L2)Cl₂ (Co2), Co(L3)Cl₂ (Co3), and Co(L4)Cl₂ (Co4) in good yields. These complexes are structurally characterized using FT-IR spectroscopy, mass spectrometry, magnetic moment measurements, elemental analysis, and single crystal X-ray diffraction for Co1 and Co4. The molecular structures of Co1 and Co4 confirm the formation of tridentate N^N^N and bidentate N^N bound monometallic complexes, respectively. Density functional theory calculations show the Co2 is most shielded (92.72%), while Co4 is the least shielded (74.11%). On the other hand, complex Fe1 displays the lowest NBO charge of 1.163, while Co2 has the highest NBO charge of 1.358. Activation of the complexes with either EtAlCl₂ or MAO cocatalysts forms active catalysts in ethylene oligomerization reactions to afford mainly ethylene dimers (C₄) and trimers (C₆). The catalytic performance of these complexes is influenced by the nature of the coordinated ligand and metal atom and is largely controlled by the relative stability of the active species. The tridentate complex Co1 (5.32 × 10⁴ g (product) mol⁻¹Co h⁻¹) is more active than the bidentate bound analogs Co3 and Co4, while catalyst Fe1 displays the highest catalytic activity of 9.66 × 10⁴ g (product) mol⁻¹Fe h.⁻¹