NXAl(3)+ (X = N, P, As): penta-atomic planar tetracoordinate nitrogen with N-X multiple bonding

Planar tetracoordinate nitrogen (ptN) has been successfully extended as a new branch of planar chemistry. As the simplest type of ptN, penta-atomic species (pptN, e.g., NAl(4)(-)) are known to have a "delocalized" molecular environment with a single bond between the central N and the ligand. In this paper, through an extensive isomeric search of a series of group V-based systems NXAl(3)(+) (X = N, P and As) in both singlet and triplet electronic states at the B3LYP/6-311+G(d) level, we report a class of novel pptN with unique chemical bonding, i.e., the central nitrogen and the connected ligand X (X = N, P and As) effectively form a highly "localized" N-X multiple bond, as confirmed by the aug-cc-pVTZ-B3LYP and MP2 calculations. The high-level CCSD(T)/aug-cc-pVTZ energetic calculations show that the three pptN species each have appreciable kinetic stability against structural transformation and fragmentation, which is confirmed by the Born-Oppenheimer molecular dynamics calculations. Possible formation pathways of the three pptNs are discussed. In particular, the pptN isomer with X = P, i.e., NPAl(3)(+), is the global minimum, making the pptN-based NPAl(3)(+) the most accessible via mass spectroscopic characterization. The present work demonstrates that the frequently used concept "localization vs. delocalization" in organic chemistry can also be transplanted to exotic planar chemistries like pptN.