Systematic study of the substitution effect on the tetrel bond between 1,4-diazabicyclo[2.2.2]octane and TH₃X

A tetrel bond was characterized in the complexes of 1,4-diazabicyclo[2.2.2]octane (DABCO) with TH₃X (T = C, Si, Ge; X= -Me, -H, -OH, -NH₂, -F, -Cl, -Br, -I, -CN, -NO₂). DABCO engages in a weak tetrel bond with CH₃X but a stronger one with SiH₃X and GeH₃X. SiH₃X is favorable to bind with DABCO relative to GeH₃X, inconsistent with the magnitude of the σ-hole on the tetrel atom. The methyl group in the tetrel donor weakens the tetrel bond but an enhancing effect is found for the other substituents, particularly -NO₂. The substitution effect is also related to the nature of the tetrel atom. The halogen substitution from F to I has a weakening effect in the CH₃X complex but an enhancing effect in the SiH₃X complex and a negligible effect in the GeH₃X complex. The above abnormal results found in these complexes can be partly attributed to the charge transfer from the lone pair on the nitrogen atom of DABCO into the anti-bonding orbital σ*(T-X) of TH₃X. The stability of both SiH₃X and GeH₃X complexes is primarily controlled by electrostatic interactions and polarization.