A potential route to the formation of alkylidene ligand from two mutually cis alkene ligands: a DFT study

The possible reaction pathways have been studied for the transformation of cis-[W(CO)4(η2-C2H4)2] into alkylidene complex by density functional theory (B3LYP) calculations in order to explain how the catalytically active alkylidene species can be formed from the bis-olefin complexes of tungsten. It was found that the two-electron oxidation of cis-[W(CO)4(η2-C2H4)2] (2et) to its dicationic form cis-[W(CO)4(η2-C2H4)2]2+ (2et2+) allowed for the transformation of this complex to tungstacyclopentane [W(CO)4(C4H8)]2+ (cp2+) at lower energy (ΔH=24 and ΔG=36 kJ mol−1) than in the case of the neutral compound 2et (ΔH=105 and ΔG=108 kJ mol−1). The electronic structure and energy of the initial 2et2+, as well as the mechanism of its interconversion to the tungstacyclopentane complex cp2+ and the final butylidene complex [W(CO)4(=CHC3H7)]2+ (but2+) were determined. An activation barrier ΔH# for the formation of the cp2+ and but2+ complexes was found to be 48 and 87 kJ mol−1, respectively. The butylidene complex but2+ is lower in energy than the tungstacyclopentane complex cp2+ and remains more stable than the initial cis-bis(ethene) complex 2et2+. The structures of two potential hydrido complexes [W(CO)4H(C4H7)]2+ (Hcp2+) are also discussed. The theoretical results are compared with available experimental data, as well as the results of theoretical studies that have been carried out on the related systems.