Unveiling the n → π* interactions in main-group carbonyl complexes

n → π* interactions have been widely explored in organic compounds, biomolecules, and transition metal complexes. However, they remain largely unexplored in main-group carbonyl systems. Herein, we report n → π* interactions between oxygen-based Lewis bases and their S- and Se-analogs with the beryllium tricarbonyl and borylene dicarbonyl complexes, Be(CO)₃ and BH(CO)₂, using density functional theory. The donors interact with these complexes through the region between the two carbonyl groups, in a nearly planar geometry. Analyses based on the quantum theory of atoms in molecules (QTAIM) and the noncovalent interaction (NCI) index reveal a double-faced noncovalent interaction between the donor and the two carbonyl groups in the above complexes. These analyses further indicate that the donors do not interact with these complexes through their central atoms, Be or B. Natural bond orbital (NBO) analysis further indicates that these interactions exhibit the key characteristics of n → π* interactions, involving the delocalization of the lone-pair electron density from a donor atom into the antibonding (π*) orbital of the carbonyl groups. The calculated interaction energies, second-order perturbation energies associated with charge transfer, and the associated geometrical parameters support this electron delocalization, reinforcing its resemblance to the nucleophilic character of the electron donor typically observed in n → π* interactions.