**An Ab Initio Investigation of the Geometries and Binding Strengths of Tetrel-, Pnictogen-, and Chalcogen-Bonded Complexes of CO2, N2O, and CS2 with Simple Lewis Bases: Some Generalizations**

**Ibon Alkorta 1,\* and Anthony C. Legon 2,\***


**\*** Correspondence: ibon@iqm.csic.es (I.A.); a.c.legon@bristol.ac.uk (A.C.L.); Tel.: +44-117-331-7708 (A.C.L.)

Received: 20 August 2018; Accepted: 30 August 2018; Published: 4 September 2018

**Abstract:** Geometries, equilibrium dissociation energies (*D*e), and intermolecular stretching, quadratic force constants (*k*σ) are presented for the complexes B··· CO2, B··· N2O, and B··· CS2, where B is one of the following Lewis bases: CO, HCCH, H2S, HCN, H2O, PH3, and NH3. The geometries and force constants were calculated at the CCSD(T)/aug-cc-pVTZ level of theory, while generation of *D*e employed the CCSD(T)/CBS complete basis-set extrapolation. The non-covalent, intermolecular bond in the B··· CO2 complexes involves the interaction of the electrophilic region around the C atom of CO2 (as revealed by the molecular electrostatic surface potential (MESP) of CO2) with non-bonding or π-bonding electron pairs of B. The conclusions for the B··· N2O series are similar, but with small geometrical distortions that can be rationalized in terms of secondary interactions. The B··· CS2 series exhibits a different type of geometry that can be interpreted in terms of the interaction of the electrophilic region near one of the S atoms and centered on the C<sup>∞</sup> axis of CS2 (as revealed by the MESP) with the n-pairs or π-pairs of B. The tetrel, pnictogen, and chalcogen bonds so established in B··· CO2, B··· N2O, and B··· CS2, respectively, are rationalized in terms of some simple, electrostatically based rules previously enunciated for hydrogen- and halogen-bonded complexes, B··· HX and B··· XY. It is also shown that the dissociation energy *D*<sup>e</sup> is directly proportional to the force constant *k*σ, with a constant of proportionality identical within experimental error to that found previously for many B··· HX and B··· XY complexes.

**Keywords:** intermolecular force constants; dissociation energies; CCSD(T)/aug-cc-pVTZ calculations; non-covalent bonds
