Bonding of eta(1)-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 eta(1)-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)(eta(5)-C5H5) and trans-Ru(C=CR)-Cl(PH3)(4), with a series of substituted acetylide ligands, R = H, C6H5 and C6H4-4-NO2. The pi back-bonding ability of the ligands increases in the order R = H < C6H5 < C6H4-4-NO2, while pi 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 rr system. The nitrophenylacetylide ligand is a much weaker a donor than either of the other two. Trends in calculated ionization energies result from a subtle interplay of changes in the sigma donor, pi donor, and pi acceptor properties of the ligands and cannot be ascribed to one mechanism in isolation.