A few years back I followed a train of thought here which ended with hexacoordinate carbon, then a hypothesis rather than a demonstrated reality. That reality was recently confirmed via a crystal structure, DOI:10.5517/CCDC.CSD.CC1M71QM[cite]10.1002/anie.201608795[/cite]. Here is a similar proposal for penta-coordinate nitrogen. First, a search of the CSD (Cambridge structure database) for such nitrogen.
AbstractIn contrast to the well‐known 2‐norbornyl cation, the structure of which was a matter of long debate until its pentacoordinated nature was recently proven by an X‐ray structure, the pentagonal‐pyramidal dication of hexamethylbenzene has received considerably less attention. This species was first prepared by Hogeveen in 1973 at low temperatures in magic acid (HSO3F/SbF5), for which he proposed a non‐classical structure (containing a hexacoordinated carbon) based on NMR spectroscopy and reactivity studies, but no X‐ray crystal structure has been reported. C6(CH3)62+ can be obtained through the dissolution of hexamethyl Dewar benzene epoxide in HSO3F/SbF5 and crystallized as the SbF6− salt upon addition of excess anhydrous hydrogen fluoride. The crystal structure of C6(CH3)62+ (SbF6−)2⋅HSO3F confirms the pentagonal pyramidal structure of the dication. The apical carbon is bound to one methyl group (distance 1.479(3) Å) and to the five basal carbon atoms (distances 1.694(2)–1.715(3) Å).
AbstractAza‐closo‐decaboran(10), NB9H10, kristallisiert in der Raumgruppe P21/n. Das NB9‐Gerüst bildet ein verzerrtes zweifach überdachtes tetragonales Antiprisma mit dem N‐Atom in axialer Position. Die Strukturdaten werden mit denen von Aza‐closo‐dodecaboran(12), NB11H12, verglichen.