Ligand non-innocence allows isolation of a neutral and terminal niobium nitride.

Complex (PNP)NbCl(N[Bu]Ar) (1) (PNP = N[2-PPr-4-methylphenyl]; Ar = 3,5-MeCH) reacts with one equiv. of NaN to form a mixture of (PNPN)NbCl(N[Bu]Ar) (2) and (PNP)NbN(N[Bu]Ar) (3), both of which have been spectroscopically and crystallographically characterized, including N isotopic labelling studies. Complex 3 represents the first structurally characterized example of a neutral and mononuclear Nb nitride. Independent studies established 3 to form two-electron reduction of 2, whereas oxidation of 3 by two-electrons reversed the process. Computational studies suggest the transmetallation step to produce the intermediate [(PNP)NbCl(N)(N[Bu]Ar)] (A) which extrudes N to form the phosphinimide [(PNPN)NbCl(N[Bu]Ar)] (B) followed by disproportionation to 2 and low-valent [(PNPN)Nb(N[Bu]Ar)] (C). The latter then undergoes intramolecular N-atom transfer to form the nitride moiety in 3.