Ab initio calculations have been performed to study the structures, binding energies, and bonding properties of the hemi-bonded binary complexes (XH2P···NH2Y)(+) with the substituents X and Y being H, F, Cl, Br, NH2, CH3, and OH. The P···N interactions in these open-shelled systems have typical pnicogen bond characteristics but much stronger than the usual pnicogen bonds in closed-shell systems. This P···N bond can be strengthened by an electron-withdrawing substituent X or an electron-donating substituent Y, the bonding energy varies from 17 kcal mol(-1) of (CH3H2P···NH2F)(+) to 54 kcal mol(-1) of (FH2P···NH2CH3)(+). A nearly linear X-P···N arrangement is required by the pnicogen bond P···N and results in a strong hyperconjugation and charge transfer from the N lone pair to the X-P σ* antibond orbital for α spin, the P···N interaction is described as a single-electron σ bond of β spin. The AIM and NBO analyses revealed that the P···N bonds in the majority of the hemi-bonded complexes are partly covalent in nature. Graphical Abstract The P···N interactions in the open-shelled systems (XH2P···NH2Y)(+) (X, Y=H, F, Cl, Br, NH2, CH3, OH) with bonding energy of 17~54 kcal mol(-1) have typical pnicogen bond characteristics but much stronger than the usual pnicogen bonds in closed-shell systems. This P···N bond can be strengthened by an electron-withdrawing substituent X or an electron-donating substituent Y.

The monocyclic compounds (BRg)(D), (BRg)(D), (BRg)(D) and (BRg)(D) formed between boron and rare gases Rg (He-Rn) are theoretically predicted to be stable structures and have π-aromaticity with a delocalized nc-2e π-system. For heavier rare gases Ar-Rn, the B-Rg bond energy is quite high and ranges from 15 to 96 kcal mol, increasing with the ring size and the atomic number of rare gases; the B-Rg bond length is close to the sum of covalent radii of B and Rg atoms; NBO and AIM analyses show that the B-Rg bonds for Ar-Rn have a typical covalent character. The B-Rg bond is stabilized mainly by σ-donation from the valence p orbital of Rg to the vacant valence orbital of the boron ring. We searched for a large number of isomers for the systems of Ar and found that the titled monocyclic compounds (BAr)(D), (BAr)(D) and (BAr)(D) should be global energy minima. For (BAr) the global energy minimum is an octahedral caged structure, but the titled monocyclic compound is the secondary stable local energy minimum. The energy and thermodynamic stability of the ring BRg cations indicate that these rare gas compounds may be viable species in experiments.

A new series of stable noble gas-Lewis acid compounds NgBeH3BeR, NgBeH3BR(+), and NgBH3BR(2+) (R = F, H, CH3, Ng = He-Rn) with three 3c-2e H-bridged bonds have been predicted by use of the PBE0 and MP2 methods. The Ng-Be/B bonds are strong and have large binding energies 35-130, 9-38, and 4-13 kcal/mol for the doubly charged cations, singly charged cations, and neutral molecules, respectively. The binding energy and strength of the Ng-Be/B bonds increase largely from He to Rn but are insensitive to electronegativity of the substituent R. The Ng-B bonds in NgBH3BR(2+) should be typical covalent bonds and the Ng-Be bonds in NgBeH3BR(+) for heavy Ng atoms Kr, Xe, and Rn have some covalent character. The three bridging-H atoms have characteristic infrared vibrational modes with large IR intensity to be detected in spectroscopy experiments.

A new series of divalent boron‐rare gas cations normalB4Rgn2+(Rg = He ∼ Rn, n = 1–4) have been predicted theoretically at the B3LYP, MP2, and CCSD(T) levels to present the structures, stability, charge distributions, bond natures, and aromaticity. The RgB bond energies are quite large for heavy rare gases and increase with the size of the Rg atom. Because of steric hindrance new Rg atoms introduced to the B4 ring will weaken the RgB bond. Thus in normalB4Rn2+ the RgB bond has the largest binding energy 90–100 kcal/mol. p‐ normalB4Rg22+ has a slightly shorter RgB bond length and a larger bond energy than o‐ normalB4Rg22+. NBO and AIM analyses indicate that for the heavy Rg atoms Ar ∼ Rn the BRg bonds have character of typical covalent bonds. The energy decomposition analysis shows that the σ‐donation from rare gases to the boron ring is the major contribution to the RgB bonding. Adaptive natural density partitioning and nuclear‐independent chemical shift analyses suggest that both normalB42+ and normalB4Rgn2+ have obvious aromaticity.