The pi-backbonding interactions illustrated below occur between the t2g d-orbital set and the CO π* antibonding orbitals. The degenerate dxz and dyz backbonding interactions with both axial and equitorial CO ligands is the HOMO-15. More total delocalization occurs onto the axial CO antibonding orbital than does the equatorial, which is thought to rationalize the shorter Mn-C bond length.The primary Mn-Mn σ-bonding orbital is composed of two dz2 orbitals, represented by the HOMO-9.
The pi-backbonding interactions depicted below involve the t2g d-orbital set interacting with the CO π* antibonding orbitals. The degenerate dxz and dyz backbonding interactions with both axial and equatorial CO ligands are the HOMO-15. More total delocalization occurs onto the axial CO antibonding orbital than does the equatorial, which is thought to explain the shorter Mn-C bond length. The primary Mn-Mn σ-bonding orbital is composed of two dz2 orbitals, represented by the HOMO-9.