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 η 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)(PH 3 ) 2 (η 5 -C 5 H 5 ) and trans -Ru(C⋮CR)Cl(PH 3 ) 4, with a series of substituted acetylide ligands, R = H, C 6 H 5 and C 6 H 4 -4-NO 2 . The π back-bonding ability of the ligands increases in the order R = H < C 6 H 5 < C 6 H 4 -4-NO 2, while π donor properties vary as R = H < C 6 H 4 -4-NO 2 < C 6 H 5 . 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.