a For each molecule, the calculated Vmax or Vmin value refers to the atom marked with asterisk in the table.
b ΔVmin refers to the changes of the most negative MEP (Vmin) on the free N atom in the binary complex relative to that of the corresponding monomer (CN-Ph-CN* or Br-Ph-CN*).
As shown in Figure 1, the two CN substituents in CN-Ph-CN make it to be a typical Lewis base due to its withdrawing nature. When R is substituted by Br atom, the MEPs maps of Br-Ph-CN possess a σ-hole on the extensions of the C-Br bond, which indicates that Br-Ph-CN can act as both Lewis acid and Lewis base. The Vmax and Vmin in these isolated monomers are gathered in Table 4. For the halogen-containing compounds, there are small positive electrostatic potential caps (σ-holes) on the outermost portion along the molecular axis, and these σ-holes represent the potential interaction sites with the Lewis base CN-Ph-CN. The calculated Vmax values for the halogen compounds ranges from 12.99 to 54.37 kcal/mol. For dihalogen compounds, the σ-hole values become more positive in the order F2<BrCl*<Cl2<Br2<ClBr*<FCl*<FBr*, and the atom which interact with CN-Ph-CN is denoted by an asterisk. As for BH3 and BF3, two positive electrostatic potential regions exist along the vertical direction of the molecular plane (π-hole), corresponding to the location of the emptyp orbital of the B atom, and thus a favorable triel bond with a Lewis base is expected for BH3 and BF3. The value of Vmax is larger in BF3 than that in BH3, which is in agreement with the earlier studies [44]. For the four nitrogen bases, blue regions with negative MEPs are found on the surface of the N atom, and the values of Vmin,Nincrease in the order HCN(sp )<NH2CH3(sp 3) <(≈)NHCH2(sp 2)<NH3(sp 3), which is in accordance with the result obtained by Li et al .[57]

3.2.2 Geometries and Interactions of the Ternary complexes

Here, we will discuss the issue that when introducing the halogen or triel bond into the original binary complex, how the pnicogen bond will be affected by the interplay of these interactions. The optimized structures of some representative ternary complexes are plotted in Figure 8. The geometrical parameters and the interaction energies of the ternary and binary complexes are summarized in Table 5 and Table S1, respectively. For the trimolecular complexes X…CN-Ph-CN…PO2F, when X is the halides compounds (F2, Cl2, Br2, FCl, FBr, BrCl, ClBr, FCN, ClCN, BrCN), the free N atom in CN-Ph-CN interacts with the halides along the extension of C-N bond, forming halogen bonds, and when X is the boron-containing compound (BH3, BF3), the B…N(C) triel bonds will be formed. As for the ternary complexes of Y…Br-Ph-CN…PO2F, the Br atom interacts with the N-bases denoted by Y, including NH3, NH2CH3, NHCH2, and HCN, with the formation of the N…Br halogen bond. The geometry of the P…N pnicogen bond is similar to that in the binary complexes, and the only difference is the P…N binding distance and the F-P-O-O dihedral angle, which indicate the strength of the pnicogen bond and the geometrical deformation upon formation of the ternary complexes, respectively.