A set of structurally modulated salicylaldimine cobalt(II) complexes have been synthesized and utilized them for ethylene oligomerizations in combination with various organoaluminum cocatalysts. All catalyst systems yielded alpha-olefins with butenes as major product. The catalytic activity and product distribution were affected by the catalytic structure and reaction conditions. The formation of active species in these complexes under oligomerization conditions was investigated by UV-VIS spectroscopy and the results matched well with oligomerization results.