Zirconium complexes of two dianionic amine bis(phenolate) ligands have been synthesized, their X-ray structures solved, and their activity as hex-1-ene polymerization catalysts studied; upon treatment with B(C₆F₅)₃, an octahedral [ONNO]Zr(CH₂Ph)₂ complex, having an extra N-donor group on a side arm, shows exceptionally high polymerization activity and yields a high molecular weight poly(hex-1-ene), whereas a related pentacoordinate [ONO]Zr(CH₂Ph)₂ complex, having no extra donor group, shows only poor activity as a polymerization catalyst.