Triel Bonds with Methyl Groups as Electron Donors. A Pentacoordinate Carbon Atom

The triel bond (TrB) formed between Be(CH₃)₂/Mg(CH₃)₂ and TrX₃ (Tr=B, Al, and Ga; X=H, F, Cl, Br, and I) is investigated via the MP2/aug-cc-pVTZ(PP) quantum chemical protocol. The C atoms of the methyl groups in M(CH₃)₂ are characterized by a negative electrostatic potential and act as an electron donor in a triel bond with the π-hole above the Tr atom of planar TrX₃. The interaction energy spans a wide range between -2 and -69 kcal/mol. Mg(CH₃)₂ forms a stronger TrB than does Be(CH₃)₂, which comports with the more negative electrostatic potential on its methyl groups. Some of the complexes involving Mg display a high degree of transfer of the methyl group from Mg to Tr, which is accompanied by an inversion of the bridging methyl and a sizable pyramidalization of the TrX₃ unit. The geometries of these complexes have the properties of the long sought pentacoordinate C which has eluded identification and characterization in the past.