A series of α-aminopyridines in the form of (2,6-C₆H₃N)(R¹)(CHR²NR³R⁴) (R¹ = R² = H R³ = H R⁴ = ⁱPr (L1a), R⁴ = ^tBu (L1b), R⁴ = Ph (L1c), R⁴ = 2,6-Me₂C₆H₃ (L1d), R⁴ = 2,6-ⁱPr₂C₆H₃ (L1e), R¹ = R² = H R³ = R⁴ = Et (L1f), R¹ = H R² = Me R³ = H R⁴ = ⁱPr (L2a), R⁴ = Ph (L2c), R⁴ = 2,6-Me₂C₆H₃ (L2d), R⁴ = 2,6-ⁱPr₂C₆H₃ (L2e), R¹ = Me R² = H R³ = H R⁴ = 2,6-ⁱPr₂C₆H₃ (L3e)) and β-aminopyridines in the form of (2-C₆H₄N)(CH₂CH₂NR¹R²) (R¹ = H R² = ⁱPr (4a), R² = ^tBu (L4b), R¹ = R² = Et (L4f)) have been prepared. Their corresponding halonickel complexes 1a–4f are synthesized by ligand substitution from (DME)NiBr₂ and the molecular structures are characterized. Four types of coordination modes include four-coordinate mononuclear species with one ligand, five-coordinate mononuclear species with two ligands, five-coordinate dinuclear species with two ligands, and a six-coordinate polymeric framework were determined by X-ray crystallography. Using methylaluminoxanes (MAO) as the activator, the nickel complexes can catalyze ethylene polymerization under moderate pressure and ambient temperature. The activity reaches 10⁵ g PE mol⁻¹ Ni h. The PE products with high branching and high crystallinity have M_n ∼ 10³ with PDI < 2.