Competition between tetrel bond and pnicogen bond in complexes of TX3-ZX2 and NH3.

The complexes formed between TX3-ZX2 (T = C, Si, Ge; Z = P, As, Sb; X = F, Cl) and NH3 were studied at the MP2/aug-cc-pVTZ(PP) level. For each TX3-ZX2, two types of complex were obtained. For CX3-ZX2, NH3 is inclined to approach the σ-hole on the Z atom, forming a pnicogen bond. For TX3-ZX2 (T = Si and Ge), however, the base favors engaging in a tetrel bond with the σ-hole on the T atom although the corresponding pnicogen-bonded complex is also stable. When NH3 approaches the CX3 terminal of CX3-ZX2, weak interactions are observed that may be classified as van der Waals interactions. The relative stability of both types of complexes is not affected by the substituent X. The tetrel bond is very strong and the largest interaction energy is up to -144 kJ mol-1. Dispersion is dominant in the weak van der Waals complexes, while tetrel- and pnicogen-bonded complexes are dominated by electrostatic interactions, with comparable contributions from polarization.