Nitrogen versus carbon in planar pentacoordinate environments supported by Be5Hn rings

Abstract

NBe₅H_nⁿ⁻³ (n = 0–5) (0A–5A) species with a novel planar pentacoordinate nitrogen (ppN) were designed by the isoelectronic substitution of the C atom in planar pentacoordinate carbon (ppC) species CBe₅H_nⁿ⁻⁴ (n = 0–5) with an N atom. The highly flexible H atoms found in ppC species CBe₅H₂²⁻ and CBe₅H₃⁻ were fixed upon the nitrogen substitution, as mirrored by the non-flexible H atoms in their ppN analogues NBe₅H₂⁻ (2A) and NBe₅H₃ (3A). Moreover, the N atom was found to fit the H-surrounded Be₅ rings better than the C atom because the ppC species CBe₅H₄ and CBe₅H₅⁺ adopted non-planar structures due to size-mismatch between the C atom and the H-surrounded Be₅ ring, but their ppN analogues NBe₅H₄⁺ (4A) and NBe₅H₅²⁺ (5A) adopted perfect planar structures. The electronic structure analyses revealed that the N atoms in 0A–5A were involved in four doubly occupied orbitals, including three six-center two-electron (6c-2e) σ bonds and one 6c-2e π bond. Therefore, these ppN species not only obey the octet rule, but also possess the interesting σ and π double aromaticity, which contributes to the stabilization. Consequently, 2A, 4A, and 5A are charged kinetically viable global energy minima, and are suitable for the gas phase generation and spectroscopic characterization.