Symmetrical P₄ cleavage at cobalt half sandwich complexes [(η⁵-C₅H₅)Co(L)] (L = CO, NHC)-a computational case study on the mechanism of symmetrical P₄ degradation to P₂ ligands

A full theoretical mechanistic investigation on the symmetrical cleavage of P₄ at the active complex fragments [(η⁵-C ₅H₅)Co(L)] (L = CO, ⁱPr₂Im; ⁱPr₂Im = 1,3-di-iso-propylimidazolin-2-ylidene), which results in the formation of the complex [{(η⁵-C₅H ₅)Co}₂(μ,η^2:2-P₂) ₂] 9, is presented. The overall reaction mechanism is a complex, multistep process. Rate-determining steps of the reaction sequence are two consecutive dissociations of the co-ligands L, which induce the decisive structural rearrangements of the P₄ unit. The choice of the co-ligand L (= CO, ⁱPr₂Im) influences the kinetic barrier as well as the energy balance of the overall reaction path significantly. The calculations further reveal a strong influence of the entropic effect on the overall reaction. As a consequence, the energy balance of the overall formation of 9 starting from [(η⁵-C₅H₅)Co(CO)] precursors is almost thermoneutral and has to overcome high kinetic barriers, whereas the reaction starting from [(η⁵-C₅H ₅)Co(ⁱPr₂Im)] precursors is exothermic, featuring lower transition barriers with stabilized intermediates. From the direct comparison of both reaction coordinates it seems that the entropic effect of the co-ligands is even stronger than their electronic influence, as for both investigated systems the reactions' energy profiles are almost identical up to intermediate [{(η⁵-C₅H₅)Co(L)} ₂(μ,η^2:2-P₄)] 5 (L = CO, ⁱPr₂Im). After the formation of 5, the first CO dissociation step renders the reaction endothermic for L = CO, whereas in the case of ⁱPr₂Im dissociation the reaction progresses exothermically. Energy decomposition analysis and fragment analysis provide a picture of the bonding mechanisms between the metal complex fragments and P ₄ in the case of the most significant intermediates and the final product.