Competition between a Tetrel and Halogen Bond to a Common Lewis Acid

The T and X atoms of TF₃X (T = C, Si, Ge, Sn; X = Cl, Br, I) can engage in a tetrel or halogen bond, respectively, with an approaching NH₃ base. With the exception of T = C, the tetrel bonds are considerably stronger than the halogen bond regardless of the nature of T or X. Because both bonds involve electron acceptance by the central TF₃X Lewis acid, there is a negative cooperativity between these two bonds when both are present. The halogen bond is weakened much more than is the tetrel bond and is in fact completely disrupted in some cases. Replacement of the three F substituents on TF₃X by H attenuates the σ-holes on both T and X atoms, weakens and stretches both bonds, and eliminates any halogen bonds involving Cl. Even in those cases where a halogen bond does occur in the dimer, this bond cannot withstand the presence of a simultaneous tetrel bond and so disappears.