Two possible reaction pathways for the formation of a ruthenium carbene complex by addition of acetylene to [RuH₂Cl₂(PH₃)₂]: A quantum chemical study

[RuH₂Cl₂(PⁱPr₃)₂] reacts with terminal alkynes to give the vinylidene complex [RuCl₂(=C=CHR)(PⁱPr₃)₂]. As a side product the carbene complex [RuCl₂(=CHR)(PⁱPr₃)₂] is formed. The formation of the vinylidene compound has been studied widely and is well understood whereas the reaction mechanism that leads to the carbene complex is still unclear. We have studied two possible reaction paths at the B3LYP level of theory: on the one hand the addition of acetylene and two subsequent 1,3-H shifts from the metal center to the C2 carbon of the acetylene ligand; on the other hand the dissociation of HCl from the starting compound, rearrangement of acetylene to vinyl and the formation of the carbene by addition of HCl. Both reaction paths have been found to be possible. The former can be understood as a 1,3-H shift followed by a 1,2-H shift due to the unusual η² coordination mode of the vinyl intermediate. The latter proceeds via protonation of the vinyl ligand and addition of Cl^- to the metal center.