Atmospheric N-2 is activated by two transient Nb(III) "(PNP) NbCl2"(PNP- = N[2-P(CHMe2)(2)-4-methylphenyl](2)) fragments to form the bridging diimido [(PNP)NbCl2]2(mu-N2) (1). Complex 1 can also be independently synthesized from Nb(IV) and Nb(V) precursors via one-electron and transmetalation reactions, respectively. In the presence of azobenzene, the transient Nb (III) intermediate, prepared from Li(PNP)and NbCl3(DME) (DME = dimethoxyethane) under Ar, cleaves the N=N bond via a metal-ligand cooperative four-electron reduction to form niobium imide and phosphoranimine functionalities. Structural studies are presented and discussed for various Nb systems bearing the pincer-type framework PNP as well as the N-2 and azobenzene activated products. Theoretical studies addressing the Nb-N-2-Nb core in 1 are also presented.