DFT modeling of the active centers formation in the catalytic system LTiCl2-Bu2Mg-Et2AlCl for alkene polymerization, where Lisa bidentate ligand of saligenin type, suggests the three-step mechanism of this process. This mechanism includes the addition of the most probable alkylating agent, RMg(mu-Cl)(2)AlR2 or RMg(mu-Cl)(2)MgR (R = alkyl), to LTiCl2 with the formation of the trinuclear heterocomplex with the alkyl bridge bond Mg-C-Ti, followed by its two-step isomerization into the active center via the beta-agostic intermediate. The free energy changes at the stages of the addition and isomerization are negative; the maximal energy barrier on the reaction pathway is small. In the Mg-free system LTiCl2-AlR3, the isomerization proceeds through the single energy barrier with a significantly higher amplitude. This could be the reason for a relatively high activity of the Mg-containing system and inactivity of the Mg-free system. (C) 2016 Elsevier B.V. All rights reserved.