Revisiting the Methyl Group as a Nonconventional Electron Donor in Triel, Tetrel, Pnictogen, Chalcogen, and Halogen Bonding

Quantum chemical calculations probe the characteristics of noncovalent interactions formed between a methyl group of MgMe₂ and a diverse set of p-block Lewis acids spanning the triel, tetrel, pnictogen, chalcogen, and halogen families. High-level ab initio computations show the absence of a formal lone pair on the C. Nonetheless, the highly polarized Mg-C bonds make the C a strong enough electron donor such that it can engage effectively in σ- and π-hole bonding interactions. The binding affinities and interaction characteristics vary markedly with the central atom identity, substituent effects, and molecular geometry of the Lewis acids. Triel compounds exhibit the strongest binding, often accompanied by significant structural reorganization and partial covalent character, exemplified by the stabilization of pentacoordinate carbon species reminiscent of S_N2 transition states. Conversely, tetrel, pnictogen, chalcogen, and halogen bonds cover a continuum from weak electrostatic and dispersion-dominated interactions to more heavily covalent bonding as the donor-acceptor interactions intensify.