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 Fe2+ 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) delta-back-donation, while the most important orbital contribution in ferrocene comes from the Fe<--Cp-2 pi-donation. The larger contributions of the e(1g)(pi) orbitals in ferrocene are caused by better energy matching rather than better overlapping of the interacting orbitals.