Octahedral group 4 bisphenolate ether complexes, activated by methylaluminoxane, are highly active and stereospecific α-olefin polymerization catalysts. X-ray crystallographic analysis reveals the Zr and Hf complexes to be closely isostructural; the bond lengths of the Hf complex are slightly shorter, but the maximum deviation is only 0.062 A. Despite the structural similarity of the Hf and Zr complexes, the Hf complexes generate more highly stereoselective catalysts. In addition to the influence of the transition metal, the structure of the ligand has a large influence on the stereospecificity. Bis-tert-butyl phenyl substituted complexes of Hf and Zr, when activated by MAO at 50-80 °C, generate high molecular weight polypropylene (Mn = 130000-360000 g/mol) with isotacticities [mmmm] > 97% and melting points as high as 165 °C.