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

Abstract

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. Show more