Abstract Series of 2-benzoxazole-1,10-phenanthrolines ( L1– L4) and 2-oxazoline-1,10-phenanthrolines ( L5– L8) were synthesized and used as tridentate N^N^N ligands in coordinating with metal (nickel, cobalt or iron) chlorides. Their metal complexes, nickel(II) ( Ni1– Ni8), cobalt(II) ( Co1– Co8) and iron(II) ( Fe1– Fe8), were characterized by elemental and IR spectroscopic analyses. The molecular structures of the ligand L2 and the complexes Ni3, Co1, Co3 and Fe2 have been determined by the single-crystal crystallography. The nickel complex Ni3 and iron complex Fe2 display an octahedral geometry, whereas cobalt complex Co1 is with a distorted bipyramidal geometry and Co3 as square pyramidal geometry. At 10 atm ethylene, all the complexes showed good activities in ethylene dimerization upon activation with appropriate aluminum cocatalysts; the nickel complexes gave the activity up to 3.11 × 10 6 g mol −1(Ni) h −1 upon activation with diethylaluminum chloride (Et 2AlCl), meanwhile the cobalt and iron complexes showed activities up to 1.51 × 10 6 g mol −1(Co) h −1 and 1.89 × 10 6 g mol −1(Fe) h −1, individually, upon activation with modified methylaluminoxane (MMAO).