Reactivity of the M-(η2-alkyne) bond [M = Cr, W]: A kinetic and DFT study

The displacement of η2-coordinated 1-hexyne and 3-hexyne by 2-picoline from the Cr(CO)5, BzCr(CO)2 and W(CO)5 fragments was studied. For the Cr systems, the data is consistent with a dissociative mechanism of alkyne displacement from the metal center. For W(CO)5(η2-1-hexyne), the alkyne displacement follows a largely associative mechanism. The bond dissociation enthalpies obtained from the kinetic analysis are in good agreement with the values obtained by detailed DFT calculations. The calculations indicate that the energy required for the steric reorganization of the alkyne ligand prior to binding with the metal is an important factor in the determination of the overall metal–(η2-alkyne) bond strength.