New Zr and Hf complexes bearing either a tridentate monoanionic 2-tert-butyl-6-[(quinolin-8-ylamino) methyl]phenolate [N,N,O-] or a dianionic 2-tert-butyl-6-[(quinolin-8 ylamido)methyl]phenolate [N,N-,O-] ligand have been synthesized and characterized. The geometries of the zirconium complexes have been studied using DFT methods. The complexes were tested as catalysts in the polymerization of ethylene and alpha-olefins after activation with methylalumoxane, resulting in the production, with moderate activities, of high molecular weight linear polyethylene and prevailingly isotactic polypropylene, poly(1-butene), poly(1-pentene) and poly(1-hexene). The content of isotactic pentads of these poly(alpha-olefins) increases while the size of the monomer increases, ranging between 50% for polypropylene and 99% for poly(1-hexene). (C) 2011 Elsevier B.V. All rights reserved.