Nitrogen versus carbon in planar pentacoordinate environments supported by Be5Hn rings

NBe5Hnn−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 CBe5Hnn−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.