Tetrel bond of pseudohalide anions with XH₃F (X = C, Si, Ge, and Sn) and its role in S_N2 reaction

The complexes of XH₃F$⋯N3−$/OCN⁻/SCN⁻ (X = C, Si, Ge, and Sn) have been investigated at the MP2/aug-cc-pVTZ(PP) level. The σ-hole of X atom in XH₃F acts as a Lewis acid forming a tetrel bond with pseudohalide anions. Interaction energies of these complexes vary from −8 to −50 kcal/mol, mainly depending on the nature of X and pseudohalide anions. Charge transfer from N/O/S lone pair to X–F and X–H $σ*$ orbitals results in the stabilization of these complexes, and the former orbital interaction is responsible for the large elongation of X–F bond length and the remarkable red shift of its stretch vibration. The tetrel bond in the complexes of XH₃F (X = Si, Ge, and Sn) exhibits a significant degree of covalency with XH₃F distorted significantly in these complexes. A breakdown of the individual forces involved attributes the stability of the interaction to mainly electrostatic energy, with a relatively large contribution from polarization. The transition state structures that connect the two minima for CH₃Br$⋯N3−$ complex have been localized and characterized. The energetic, geometrical, and topological parameters of the complexes were analyzed in the different stages of the S_N2 reaction $N3−$ + CH₃Br → Br⁻ + CH₃N₃.