Nitrogen versus carbon in planar pentacoordinate environments supported by Be(5)H(n) rings

NBe(5)H(n)(n−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 CBe(5)H(n)(n−4) (n = 0–5) with an N atom. The highly flexible H atoms found in ppC species CBe(5)H(2)(2−) and CBe(5)H(3)(−) were fixed upon the nitrogen substitution, as mirrored by the non-flexible H atoms in their ppN analogues NBe(5)H(2)(−) (2A) and NBe(5)H(3) (3A). Moreover, the N atom was found to fit the H-surrounded Be(5) rings better than the C atom because the ppC species CBe(5)H(4) and CBe(5)H(5)(+) adopted non-planar structures due to size-mismatch between the C atom and the H-surrounded Be(5) ring, but their ppN analogues NBe(5)H(4)(+) (4A) and NBe(5)H(5)(2+) (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.