The synthesis and structural X-ray diffraction studies for some benzamidinate ligations and several group 4 benzamidinate complexes are presented. The use of the cis-octahedral C-2-symmetry compounds was studied to shed light on the conceptual applicability of these complexes as potential catalysts for the stereoregular polymerization of propylene. We demonstrate that the stereoregular polymerization of propylene catalyzed by early-transition metal octahedral benzamiclinate complexes, activated with either MAO or B(C6F5)(3) as cocatalysts, can be modulated by pressure (from atactic to isotactic through elastomers). The different effects in the polymerization process such as the nature of solvent or cocatalyst, temperature, pressure, molar ratio catalyst: cocatalyst, and the relationship between the symmetry of the complex and the polymer microstructure have been investigated. When the complex [4-CH3-C6H4C(NTMS)(2)](2)ZrMe2 (9) was activated with MAO, it was found to be a good catalyst for the polymerization of propylene, at atmospheric pressure, producing an oily polymer resembling an atactic polypropylene. Being activated with B(C6F5)3, complex 9 produces a highly isotactic (mmmm = 98%) product. Likewise, when the polymerization of propylene was performed with complex 9 and MAO at high pressure (liquid propylene), a highly stereoregular polymer was also obtained. Larger activities and stereoregularities were achieved by performing the reaction in CH2Cl2 as compared to toluene. Contrary to complex 9, at atmospheric pressure the complex [4-CH3-C6H4C(NTMS)(2)](2)TiMe2 (10) is not active either in CH2Cl2 or in toluene. At high pressure, complex 10 produces elastomeric polypropylene. Activities of the isolobal complexes [C6H4C(NTMS)(2)](2)ZrMe2 (11) and [C6H4C(NTMS)(2)](2)TiMe2 (12) were found to be larger than those of complexes 9 and 10, respectively. Contrary to the structures of the elastomeric polypropylenes described in the literature, the obtained elastomers are characterized by frequent alternation of the isotactic domains with stereodefects. The stereoregular errors are formed by the intramolecular epimerization of the growing chain at the last inserted unit. The epimerization reaction was corroborated through the isomerization of alkenes.