A series of ferrous complexes (L)FeCl2 (3a−f) and cobaltous complexes (L)CoCl2 (4a−f) were prepared by the reaction of 2-(carboxylato)-6-iminopyridines 2a−f (2-COOEt-6-(2,6-R2C6H3NCCH3)C5H3N:  2a, R = CH3; 2b, R = Et; 2c, R = i-Pr; 2d, R = F; 2e, R = Cl; 2f, R = Br) with FeCl2 or CoCl2. The obtained complexes were characterized by elemental analysis and IR spectroscopy, and the solid-state structures of 2b,c, 3b,c,e,f, and 4c,f were determined by X-ray diffraction analysis. X-ray crystallographic analyses of complexes 3c,e,f and 4c,f reveal a four-coordinated distorted-tetrahedral geometry, except for complex 3b, in which the tridentate ligand is coordinated through a weak bonding between iron and the carbonyl oxygen of the ester group. The complexes were studied for ethylene oligomerization and polymerization in the presence of methylaluminoxane (MAO) under various reaction conditions. It was found that the ferrous complexes exhibit higher activities for ethylene polymerization than for oligomerization, while the cobaltous complexes show higher activities for ethylene oligomerization. In addition, the ferrous catalytic systems predominantly produce linear oligomers and polyethylene with unsaturated end groups.