Bis(benzene)chromium Is a δ-Bonded Molecule and Ferrocene Is a π-Bonded Molecule

The nature of the metal−ligand bonding in ferrocene and bis(benzene)chromium has been analyzed with the help of an energy partitioning scheme using the results of DFT calculations. The bonding analysis suggests that the Cr−Bz 2 bond is 37.9% electrostatic and 62.1% covalent. The binding interactions in ferrocene are predicted to be 51.1% electrostatic and 48.9% covalent if the charged species Fe 2+ and (Cp - ) 2 are used as interacting fragments, while they are 45.0% electrostatic and 55.0% covalent if neutral Fe and Cp 2 in the triplet states are used. The largest contributions to the orbital interactions in bis(benzene)chromium come from the Cr→Bz 2 δ-back-donation, while the most important orbital contribution in ferrocene comes from the Fe←Cp 2 π-donation. The larger contributions of the e 1g (π) orbitals in ferrocene are caused by better energy matching rather than better overlapping of the interacting orbitals.