Trends in back-bonding in the series trans-[M(C≡CR)Cl(PH₃)₄] (M = Fe, Ru, Os; R = H, Ph, C₆H₄NO₂-4)

The electronic structure of the complexes trans-[M(C≡CR)Cl(PH₃)₄] (M = Fe, Ru, Os; R = H, Ph, C₆H₄NO₂-4) has been investigated using approximate density functional theory in order to examine the M-C back-bonding interaction. For all three metal systems, the π back-bonding increases in the order R = H < Ph < C₆H₄-4-NO₂, indicating that the π acceptor character of the acetylide ligand increases with the electron-withdrawing ability of the substituent. The inclusion of relativistic effects in the calculations results in a metal dependence of Fe ∼ Ru < Os for the back-bonding, consistent with the observed trend in quadratic hyperpolarizabilities, β.