Ziegler-Natta ethylene insertion into the carbon-metal bond for a number of fragments containing vanadium on different oxidation states as well as the halogen ligands has been studied by means of DFT. It is shown that the complexation and insertion energies are strongly influenced by the charge on the transition metal atom and, to some extent, by the electronegativity of the halogen atom. Complexation energy vanes in the range -129 to -159 kJ/mol for charged species and -64 to -77 kJ/mol for neutral ones. Insertion energy follows a similar pattern and ranges from -28 to -62 and -82 to -100 kJ/mol, respectively. The calculated values are compared with the experimental results and different activities of catalysts based on VX3 (where X denotes a halogen atom) are accounted for. Discussion of certain structural features of active sites, transition states and products is also given.