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

Both coaxial and perpendicular singlet spin state structures are found for the dibenzene dimetal complexes (CH)M (M = Fe, Mn, and Cr) using density functional theory. For (CH)M (M = Fe, Mn), the coaxial structure is the lower energy structure, whereas for (CH)Cr 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 (CH)Fe, ~1.75 Å for (CH)Mn, and ~1.68 Å for (CH)Cr. Investigation into the frontier molecular orbitals suggests a formal 2 π Fe=Fe double bond in (CH)Fe, a σ + 2 π Mn≡Mn triple bond in (CH)Mn, and a σ + 2 π + δ quadruple bond in (CH)Cr. This gives each metal atom in these coaxial (CH)M (M = Fe, Mn, Cr) derivatives a 16-electron configuration suggesting an 8-orbital d p metal valence orbital manifold without the involvement of the s orbital. The coaxial (CH)M (M = Fe, Mn) derivatives have ideal sixfold D symmetry. However, distortion of coaxial (CH)Cr from D symmetry to D symmetry is observed because of involvement of only one orbital from the { d( xy), d( x − y)} set of δ symmetry of each chromium atom in the [InlineMediaObject not available: see fulltext.] formal quadruple bond. [ABSTRACT FROM AUTHOR]