Molecular orbital interpretation of the metal–metal multiple bonding in coaxial dibenzene dimetal compounds of iron, manganese, and chromium

Abstract

Both coaxial and perpendicular singlet spin state structures are found for the dibenzene dimetal complexes (C₆H₆)₂M₂ (M = Fe, Mn, and Cr) using density functional theory. For (C₆H₆)₂M₂ (M = Fe, Mn), the coaxial structure is the lower energy structure, whereas for (C₆H₆)₂Cr₂ the perpendicular structure is the lower energy structure. These coaxial structures are predicted to have very short M–M distances of ~ 1.98 Å for (C₆H₆)₂Fe₂, ~ 1.75 Å for (C₆H₆)₂Mn₂, and ~ 1.68 Å for (C₆H₆)₂Cr₂. Investigation into the frontier molecular orbitals suggests a formal 2π Fe=Fe double bond in (C₆H₆)₂Fe₂, a σ + 2π Mn:Mn triple bond in (C₆H₆)₂Mn₂, and a σ + 2π + δ quadruple bond in (C₆H₆)₂Cr₂. This gives each metal atom in these coaxial (C₆H₆)₂M₂ (M = Fe, Mn, Cr) derivatives a 16-electron configuration suggesting an 8-orbital d ⁵ p ³ metal valence orbital manifold without the involvement of the s orbital. The coaxial (C₆H₆)₂M₂ (M = Fe, Mn) derivatives have ideal sixfold D_6h symmetry. However, distortion of coaxial (C₆H₆)₂Cr₂ from D_6h symmetry to D_2h symmetry is observed because of involvement of only one orbital from the {d(xy), d(x ² - y ²)} set of δ symmetry of each chromium atom in the $Cr \equiv Cr$ formal quadruple bond.