Investigation of Tetrel Bonds in Isolated Complexes Formed Between a Lewis Acid HMX, M-O or M-S (M = Si, Ge, or Sn) and the Lewis Bases B = N, CO, HCCH, PH, CH, HCN, CS, HNC, NP, HO, and NH.

Isolated complexes of the type B⋯A in which the noncovalent interaction is a tetrel bond have been characterized by calculations at the CCSD(T)(F12c)/cc-pVDZ-F12 level. The Lewis bases B involved were N, CO, HCCH, PH, CH, HCN, CS, HNC, NP, HO and NH. 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 HMX (X = F, Cl, CN, H), in each of which the most electrophilic region was found to lie on the axis of the C 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 of each B⋯HMX complex plotted against the nucleophilicities of the Lewis bases B, were used to obtain the electrophilicity of each molecule HMX (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.