The synthesis and characterization of (benz)imidazolin-2-iminato scandium and yttrium complexes (Sc1.Sc3, Y1, and Y2) are reported herein. All of the complexes were isolated in high yields, and from single-crystal X-ray diffraction analysis, extremely short N.Mt bonds were revealed, indicating the highly nucleophilic nature of the ligands. In the presence of [Ph3C][B(C6F5)4] and Al(i-Bu)3, scandium complexes showed medium to high activities for 1-hexene polymerization, revealing the trend Sc1 > Sc3 > Sc2. More importantly, the scandium complex Sc1 was active in the copolymerization of 1-hexene with polar ω-halo-α-alkenes, demonstrating the good heteroatom tolerance of the cationic lanthanide metals that is caused by the strong electrondonating ability of these ligands. The influence of different types of comonomers on the polymerization behaviors and the microstructure of the resultant copolymer products were investigated in detail.