Strong Triel Bonds with Be as Electron Donor

A systematic theoretical study was conducted on the triel bonds (TrBs) within the TrX₃···Be(CO)₃ complexes (Tr = B, Al, Ga, In, Tl; X = H, F, Cl, Br, I). The interaction energies of these systems range between 4 and 38 kcal/mol. The TrB weakens as X becomes more electronegative in the B and Al systems, while the opposite pattern of stronger bonds is observed in the In and Tl analogues. The dominant component of the TrB is polarization energy, which arises from charge transfer from Be(CO)₃ to TrX₃. The source of the density is a confluence of CO π-bonding orbitals at the Be center that resembles a Be lone pair, and which makes the molecular electrostatic potential above the Be somewhat negative. This π-lump is paired with the highly positive π-hole above the Tr, and a large amount of charge is transferred to the empty p orbital of Tr. These factors, when considered in conjunction with large AIM measures, confer on this TrB a certain degree of covalency.