Dual Noncovalent Bonding to Planar Tetracoordinated Oxygen

The OTr₄ molecule, where Tr refers to a triel atom Al, Ga, In, or Tl, places the O at the center of a square. The region directly above the O can host the halogen X atom of FX or MeX to form a T-shaped complex. Quantum chemical calculations measure various aspects of the binding which is found to consist of two separate components. Charge can be transferred from the p_<i>z</i> orbital of O to the antibonding σ*(XF/XC) orbital in a classical halogen bond. At the same time, charge can shift in the opposite direction, from X lone pairs to empty Tr lone pair orbitals, so as to form Tr···X triel bonds to all four Tr centers. Both forms of bonding are present in all dyads, but one or the other takes precedence depending on the identity of the FX/MeX molecule and the Tr atom. FX forms much stronger bonds to OTr₄ than does MeX, with interaction energies approaching 40 kcal/mol.