Further considerations on the structure and bonding in edge-sharing bioctahedral complexes

Abstract

Extended Huckel MO calculations have been carried out on model compounds that mimic transition metal edge-sharing bioctahedral (ESBO) complexes of stoichiometry M 2X 6L 4 (X=anionic ligand with lone pairs for bridge-bonding and for π bonding, L=neutral 2-electron donor ligand), and for the corresponding mononuclear cis-MX 4L 2 system. The widening of the cis-L eq-M-L eq angle (eq=equatorial) is shown to cause the narrowing of the opposite X-M-X angle which, for the ESBO complexes, disfavors the formation of a strong metal-metal interaction. The calculations also address the importance of the M-X π interactions in metal-metal bonded dimers. The X ax (ax=axial) ligands are shown to be better π donors than the X eq ligands for the configurations d 1-d 1 through d 5-d 5, the differential between the π donating abilities in the two different positions being maximal for the d 1-d 1 configuration. This effect is proposed to be responsible for the preference of d 1-d 1 systems for the ESBO structure having all L ligands in equatorial positions, whereas the metal-metal bonded ESBO compounds of all other electronic configurations as well as all non-bonded ESBO complexes prefer the structure with two L eq on one metal and two L ax, on the other one on steric grounds. The MO model presented here is also in excellent agreement with the observed trends of M-Cl ax, M-Cl eq and M-Cl br (br=bridging) bond distances as a function of d nd- n configuration.