The mechanism of selective co-oligomerization of ethylene and 1-hexene by the catalyst [CrCl3(PNPOMe)] (a, PNPOMe = N,N-bis(bis(o-methoxyphenyl)phosphine)methylamine) has been explored in detail using the density functional theory (DFT) method. The full catalytic cycles for the formation of 1-hexene and 1-decenes were calculated on the basis of the metallacyclic mechanism, and the distribution of all decene isomers was explained by locating Gibbs free energy surfaces of various pathways, which is in good agreement with the experimental results. A spin surface crossing through a minimum energy crossing point (MECP) from a sextet to a quartet surface takes place before the formation of metallacyclopentane, which opens up a much lower energy pathway and thus facilitates the following co-oligomerization reactions. It is worth noting that β-hydrogen agostic-assisted hydrogen transfer is of crucial importance for the decomposition of the metallacycle intermediates to give 1-hexene or decenes. Moreover, an analysis of the Cr–O bond distance and NBO charges indicates the important role of a hemiliable methoxy moiety, which acts as a pendant group in the co-oligomerization of ethylene and 1-hexene by CrCl3(PNPOMe) catalyst.

To elucidate fundamental mechanistic aspects of the landmark Chevron–Phillips ethylene trimerization system, a detailed theoretical study has been carried out by DFT methods on an aluminum pyrrolyl chromium catalyst. Reaction pathways for selective ethylene oligomerization have been successfully located on the basis of the metallacycle mechanism. Consistent with experimental results, for the model system ethylene trimerization was proven to be energetically preferred in comparison to ethylene dimerization or further ring expansion toward the formation of higher α-olefins. The Cr(I/III) redox couple was found to be the most likely for the catalytic ethylene trimerization. A careful electronic configuration analysis has been conducted, and the ground state of all active species involved in the catalytic cycle is identified to be S = 3/2 except for the bare active species, which favors a high spin state of S = 5/2. The role of a pendant chlorine functionality has been investigated as well. Variable Cr–Cl bond distance and NBO charge analysis of every intermediate clearly exhibit the hemilabile behavior of the chlorine. This unique hemilability is considered to be a key factor for the selectivity toward 1-hexene formation.