Topological Analysis of the Electron Density in the Carbonyl Complexes M(CO)8 (M = Ca, Sr, Ba)

The quantum theory of atoms in molecules (QTAIM) has been applied to the recently synthesized alkaline-earth cubic Oh-symmetric complexes Ca­(CO)8 (1), Sr­(CO)8 (2), and Ba­(CO)8 (3). Theoretical calculations reveal that M–CO interactions in these complexes can be properly described as highly polar bonds, showing some features traditionally associated with transition-metal bonding, although with noticeable differences, as well. In this sense, δ­(M–C) and δ­(M···O) delocalization indexes for bonding and nonbonding interactions, electron localization funcion (ELF) analyses, source function (SF) calculations, and the interacting quantum atoms (IQA) approach, among other methodologies, produce results consistent with interactions dominated by electrostatics between the CO ligands and alkaline-earth metals, with an increasing degree of covalency on going from 1 to 3 and without any significant π-back-donation.