This work addresses a counterintuitive observation in the reactivity of the well-known ruthenium complexes [Ru(X)H(CO)(PiPr3)2], according to which the 5-coordinate chloro complex (X = Cl, 1) is less reactive toward phenylacetylene than its 6-coordinate acetate analogue (X = κO2-OC(O)Me, 3), since 3 undergoes a hydride-to-alkenyl-to-alkynyl transformation, whereas the reaction of 1 stops at the alkenyl derivative. The experimental kinetics of the key alkenyl-to-alkynyl step in the acetate complex are compared to the results of DFT calculations, which disclose the ability of the acetate not only to assist the alkyne C–H activation step via a CMD mechanism but also to subsequently deliver the proton to the alkenyl ligand. Possible consequences of this mechanistic resource connecting mutually trans ligands are briefly discussed on the basis of reported chemoselectivity changes induced by carboxylate ligands in 1-alkyne hydrosilylations catalyzed by this type of ruthenium complexes.
