Ab Initio Investigation of Tetrel Bonds in Isolated Complexes Formed Between a Lewis Acid H₃MX, M–O or M–S (M = Si, Ge, or Sn) and the Lewis Bases B = N₂, CO, HCCH, PH₃, C₂H₄, HCN, CS, HNC, NP, H₂O, and NH₃

Isolated complexes of the type B⋯A in which the noncovalent interaction is a tetrel bond have been characterized by ab initio calculations at the CCSD(T)(F12c)/cc-pVDZ-F12 level. The Lewis bases B involved were N₂, CO, HCCH, PH₃, C₂H₄, HCN, CS, HNC, NP, H₂O, and NH₃. Two types of Lewis acid A were examined, each containing one of the tetrel atoms M = Si, Ge or Sn, The Lewis acids in the first series were the H₃MX (X = F, Cl, CN, H), in each of which the most electrophilic region was found to lie on the C₃ axis of the C_3v molecules, near to the tetrel atom M. In the second series the Lewis acids were M–O and M–S. Graphs, consisting of calculated equilibrium dissociation energies D_e of each B⋯H₃MX complex plotted against the nucleophilicities NB of the Lewis bases B, were used to obtain the electrophilicity E_H3MX of each molecule H₃MX (M = Si, Ge, Sn). The molecular electrostatic surface of potentials of the molecules M–S and M–O (M = Si, Ge, Sn) revealed that many of the B⋯M-S and B⋯M−O complexes should have a tetrel bond to M in which the axis of the M–S or M–O subunit should be approximately perpendicular to the axis of the nonbonding or π-bonding electron pair carried by B, a novel type of tetrel bond confirmed by geometry optimizations of the complexes.