Metal–CO Bonding in Mononuclear Transition Metal Carbonyl Complexes

[Image: see text] DFT calculations have been carried out for coordinatively saturated neutral and charged carbonyl complexes [M(CO)(n)](q) where M is a metal atom of groups 2–10. The model compounds M(CO)(2) (M = Ca, Sr, Ba) and the experimentally observed [Ba(CO)](+) were also studied. The bonding situation has been analyzed with a variety of charge and energy partitioning approaches. It is shown that the Dewar–Chatt–Duncanson model in terms of M ← CO σ-donation and M → CO π-backdonation is a valid approach to explain the M–CO bonds and the trend of the CO stretching frequencies. The carbonyl ligands of the neutral complexes carry a negative charge, and the polarity of the M–CO bonds increases for the less electronegative metals, which is particularly strong for the group 4 and group 2 atoms. The NBO method delivers an unrealistic charge distribution in the carbonyl complexes, while the AIM approach gives physically reasonable partial charges that are consistent with the EDA-NOCV calculations and with the trend of the C–O stretching frequencies. The AdNDP method provides delocalized MOs which are very useful models for the carbonyl complexes. Deep insight into the nature of the metal–CO bonds and quantitative information about the strength of the [M] ← (CO)(8) σ-donation and [M(d)] → (CO)(8) π-backdonation visualized by the deformation densities are provided by the EDA-NOCV method. The large polarity of the M–CO π orbitals toward the CO end in the alkaline earth octacarbonyls M(CO)(8) (M = Ca, Sr, Ba) leads to small values for the delocalization indices δ(M–C) and δ(M···O) and significant overlap between adjacent CO groups, but the origin of the charge migration and the associated red-shift of the C–O stretching frequencies is the [M(d)] → (CO)(8) π-backdonation. The heavier alkaline earth metals calcium, strontium and barium use their s/d valence orbitals for covalent bonding. They are therefore to be assigned to the transition metals.