A series of tridentate cobalt complexes of the general formula [$RN(CH_2)_2N$]=An=[N(2,6-$C_6H_3iPr$)] (R=Me, Et, iPr; An=acenaphthenequinone) have been synthesized. All complexes were active towards ethylene oligomerization, showing activities exceeding $10^4$ g-oligomer/mol-$Co{cdot}h{cdot}bar$ in the presence of a methylaluminoxane co-catalyst. 1-Butene and trans-2-hexene were the major oligomer fractions obtained.