Metal-Centered Noncovalent Interactions: A Comparative Insight into π-Hole- and σ-Hole-Directed Bonds

This article investigates the π-hole-driven interactions of BeCl₂, MgCl_2, and HgCl₂, as well as the σ-hole-directed interactions of MoOF₄ and OsO₄ with substituted pyridines. Among the π-hole complexes, the Be…N beryllium bond exhibits the highest binding strength, followed by Mg…N magnesium bond and Hg…N spodium bond, whereas within the σ-hole complexes, the Mo…N wolfium bond is stronger than the Os…N osme bond. The overall binding energy (ΔE) trend follows: BeCl₂ > MgCl₂ > MoOF₄ > HgCl₂ > OsO₄, and shows a linear correlation with the deformation energy of the Lewis acids. AIM analysis reveals partial covalent character of all M…N interactions (M = metal), except MgCl₂ complexes, which are predominantly electrostatic in nature. Natural bond orbital (NBO) analysis indicates that although second-order hyperconjugation energies correlate with ΔE within each class, they are not solely determinative; notably, despite significantly higher E² values corresponding to LP(N) → LP*(M) interaction in BeCl₂ relative to MgCl₂ complexes, the difference in ΔE remains marginal due to a larger opposing exchange contribution in BeCl₂ complexes. Furthermore, while σ-hole complexes are primarily governed by LP(N) → σ*(M-O) interactions, MoOF₄ uniquely exhibits an additional LP(N) → LP*(Mo) orbital interaction. The bond (MCl/MO) elongations and the corresponding vibrational redshifts show strong linear correlations with ΔE.