Bonding of η¹-acetylide ligands to electron-rich ruthenium centers: Can electron-withdrawing ligands induce significant metal-to-ligand back-bonding?

The electronic structure of a series of η¹-acetylide complexes containing electron-rich ruthenium centers is examined using approximate density functional theory. Calculations are performed on two series of complexes, Ru(C≡CR)(PH₃)₂(η⁵-C₅H ₅) and trans-Ru(C≡CR)-Cl(PH₃)₄, with a series of substituted acetylide ligands, R = H, C₆H₅ and C₆H₄-4-NO₂. The π back-bonding ability of the ligands increases in the order R = H < C₆H₅ < Ce₆H₅ < C₆H₄-4-NO₂, while π donor properties vary as R = H < C₆H₄-4-NO₂ < C₆H₅. The cations of the complexes of the phenylacetylide ligand are therefore relatively stable, particularly when the vacancy at the metal center arises in an orbital coplanar with the phenyl π system. The nitrophenylacetylide ligand is a much weaker σ donor than either of the other two. Trends in calculated ionization energies result from a subtle interplay of changes in the σ donor, π donor, and π acceptor properties of the ligands and cannot be ascribed to one mechanism in isolation.