A quantum chemical calculation on Fe(CO)5 revealing the operation of the Dewar-Chatt-Duncanson model

A nonstandard computational scheme has been applied to calculate Fe(CO)5 with the aim to illustrate the operation of the Dewar–Chatt–Duncanson model by computation. A full configuration interaction (CI) calculation in an active space has been performed. The active space is built from naturally localized molecular orbitals (NLMOs) localized in bond regions or forming lone pairs. For selecting this active space, Weinhold's perturbation theory formulated in the natural bond orbital (NBO) space has been applied. Bonding, lone pair, and antibond NBOs exhibiting large interaction energies serve to define the active space. The actually applied active space, however, comprises NLMOs that are close in shape to the NBOs indicated by perturbation theory. Thus, a CI calculation with localized orbitals has been performed meeting the classical reasoning of chemists that is often based on local bonding concepts. The computational scheme yields the Lewis structure for Fe(CO)5 whose energy is identical to the Hartree–Fock energy. The Lewis energy comprises CO → Fe σ‐electron transfer (ET) and CO ← Fe electron back donation (BD). This Lewis energy gets lowered by localized correlation energy contributions caused by ET processes where electrons are back donated from the Fe d‐lone pairs into the CO ligands. Thus, electron correlation within the selected active space is dominated by electron BD. Energies and electron populations of the NBOs support the notion that electrons are preferentially back donated into the equatorial CO ligands. Weights of local Slater determinants, determining the correlation energy, also point to a predominant BD into the equatorial CO ligands. Correlation energy increments resulting from electron BD into single antibond orbitals of the CO ligands have been calculated. These energy quantities also demonstrate that BD into the equatorial CO ligands is more energy lowering than BD into the axial CO ligands. © 2012 Wiley Periodicals, Inc. The classic Dewar‐–Chatt‐–Duncanson (DCD) model is a qualitative theoretical device for rationalizing the bond nature in compounds where a central transition metal is surrounded by ligands. The model assumes that ligand electrons get donated to the transition metal atom. The resulting electron excess at the metal induces an electron back‐ donation from the metal to the ligands. The article shows how the DCDDewar‐Chatt‐Duncanson model is substantiated by emerging from a non‐standard ab initio calculation on Fe(CO)5.