Catalyst compositions and polymerization systems include at least one thioguanidine complex according to Formula (I): MQaX4_a (I) in which M is Ti, Zr, or Hf; a is 1 or 2; each group Q of the thioguanidine complex is a bidentate thioguanidine ligand bound to the metal center and having Formula (1a) or Formula (1b), In Formulas (1a) and (1b), each group R1, R2, and R3 is independently chosen from alkyl groups or aryl groups; each group Z1 is independently chosen from alkylene groups. If a=2, groups R3 of the two groups Q are optionally linked to each other through at least one covalent bond. Each X is covalently bonded or coordinated to the metal center and is independently chosen from alkyl groups or halides. The polymerization systems may be configured to copolymerize ethylene and α-olefins.