Several [ONXO]-type zirconium and hafnium dibenzyl complexes of amine bis(phenolate) ligands, where X is a heteroatom donor located on a pendant arm, were synthesized directly from the ligand precursors and the corresponding tetrabenzylmetal complexes in quantitative yields (X = N, O, S). All complexes exhibited remarkable activities in the polymerization of 1-hexene, yielding high-molecular-weight polymers, the highest activity being unprecedented under the conditions employed. An unexpected metal-dependent activity pattern was demonstrated. The high activity of the complexes is derived from the binding of the side-arm donor to the metal and is affected by its nature. The activity order of the zirconium complexes as a function of the side-arm donor was found to be OMe > NMe2 > SMe (5:2:1). The X-ray structures of all zirconium complexes were solved and revealed very similar binding of the [ONO] ligand cores to the metal. The distances between the side-arm donor atoms and the zirconium are relatively long (Zr-O, 2.45 Å; Zr-N, 2.59 Å; Zr-S, 2.86 Å). The X-ray structures of all hafnium complexes were solved as well and found to be closely related to those of the zirconium analogues (Hf-O, 2.45 Å; Hf-N, 2.56 Å; Hf-S, 2.84 Å). However, the activity order was found to be different in the hafnium series, namely SMe > OMe > NMe2 (3.5:1.5:0.2), the [ONSO]-type complex exhibiting the highest activity ever reported for a hafnium complex under the conditions employed. Various NMR experiments supported the notion that the side-arm donor remains attached to the metal in the reactive compounds, which exist as well-separated ion pairs.