It should be noted that, although 2 presents a very strong affinity for dinitrogen, the N 2 coordination is reversible with temperature (Figure ). Upon heating a solution of 5 in tol-d 8 to reflux in a sealed NMR tube under an argon atmosphere, the purple color slowly fades away, leading to the orange-red color characteristic of 2. Agitation of the NMR tube immediately returns the original purple colors, indicating re-coordination of the N 2 ligand. A movie about this transformation can be found in the Supporting Information section. Reversible N 2 coordination is unusual in RE dinitrogen complexes, dominated by side-on N 2 coordination. It has historically been observed in some Sm which are less reducing than non-classical Ln II complexes and thus afford weaker N 2 activation. In the case of RE end-on coordinated (N 2 ) 2-complexes, the liberation of N 2 with concomitant generation, even transiently, of Ln II species seems to be more common and has been observed both thermally 50b,50d or under UV light irradiation.