Characterizing magnesium bonds: main features of a non-covalent interaction

Compounds of formula MgX2 can interact with small Lewis bases such as halides, amines, phosphines or water. These interactions are directly related with the chemistry of very relevant compounds, such as Grignard reagents and their role in the Schlenk equilibrium. A systematic exploration of the interaction between MgCl2 and a series of Lewis bases is carried out with different theoretical approaches, along with the analysis of the topological features and the estimation of interaction and binding energies. The ability of Mg to establish non-covalent interactions, named in the literature as magnesium bonds, is compared to beryllium bonds. Magnesium bonds present topological features of closed-shell interactions (∇2ρ > 0). Interaction energies at the B3LYP/6-311 + G(3df,2p) level including BSSE correction are in a range between −36 and −125 kJ/mol, larger than many typical hydrogen bonds. Although weaker for bases such as water or ammonia, magnesium energies are larger than the beryllium ones for hydrogen halide complexes due to the presence of a neighboring hydrogen bond. Unlike beryllium bonds, interactions between lone pairs of the Lewis base and empty Mg orbitals and antibonding Mg–Cl orbitals are of equal importance, being the second ones more relevant for Lewis bases from the second row and beyond.