**Mehdi D. Esrafili \* and Parisasadat Mousavian**

Laboratory of Theoretical Chemistry, Department of Chemistry, University of Maragheh, Maragheh 5513864596, Iran; p.mosavian1327@gmail.com

**\*** Correspondence: esrafili@maragheh.ac.ir; Tel.: +98-42-1223-7955; Fax: +98-42-1227-6060

Received: 25 September 2018; Accepted: 5 October 2018; Published: 15 October 2018

**Abstract:** In recent years, noncovalent interactions involving group-14 elements of the periodic table acting as a Lewis acid center (or tetrel-bonding interactions) have attracted considerable attention due to their potential applications in supramolecular chemistry, material science and so on. The aim of the present study is to characterize the geometry, strength and bonding properties of strong tetrel-bond interactions in some charge-assisted tetrel-bonded complexes. Ab initio calculations are performed, and the results are supported by the quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) approaches. The interaction energies of the anionic tetrel-bonded complexes formed between XF3M molecule (X=F, CN; M=Si, Ge and Sn) and A− anions (A−=F−, Cl−, Br−, CN−, NC− and N3 <sup>−</sup>) vary between −16.35 and −96.30 kcal/mol. The M atom in these complexes is generally characterized by pentavalency, i.e., is hypervalent. Moreover, the QTAIM analysis confirms that the anionic tetrel-bonding interaction in these systems could be classified as a strong interaction with some covalent character. On the other hand, it is found that the tetrel-bond interactions in cationic tetrel-bonded [p-NH3(C6H4)MH3] <sup>+</sup>···Z and [p-NH3(C6F4)MH3] <sup>+</sup>···Z complexes (M=Si, Ge, Sn and Z=NH3, NH2CH3, NH2OH and NH2NH2) are characterized by a strong orbital interaction between the filled lone-pair orbital of the Lewis base and empty BD\*M-C orbital of the Lewis base. The substitution of the F atoms in the benzene ring provides a strong orbital interaction, and hence improved tetrel-bond interaction. For all charge-assisted tetrel-bonded complexes, it is seen that the formation of tetrel-bond interaction is accompanied bysignificant electron density redistribution over the interacting subunits. Finally, we provide some experimental evidence for the existence of such charge-assisted tetrel-bond interactions in crystalline phase.

**Keywords:** noncovalent interaction; tetrel-bond; *σ*-hole; electrostatic potential; ab initio
