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 A for (C6H6)2Fe2, ~1.75 A for (C6H6)2Mn2, and ~1.68 A 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 formal quadruple bond.