It should be noted that, although 2 presents a very strong affinity for dinitrogen, the N2 coordination is reversible with temperature (Figure ). Upon heating a solution of 5 in tol-d8 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 N2 ligand. A movie about this transformation can be found in the Supporting Information section. Reversible N2 coordination is unusual in RE dinitrogen complexes, dominated by side-on N2 coordination. It has historically been observed in some Sm which are less reducing than non-classical Ln II complexes and thus afford weaker N2 activation. In the case of RE end-on coordinated (N2) 2-complexes, the liberation of N2 with concomitant generation, even transiently, of Ln II species seems to be more common and has been observed both thermally 53b,53d or under UV light irradiation. 53a