The exact mechanism of the N 2 hydrogenation reactivity remains elusive to date but control experiments revealed no reaction between 3-H and N 2 at different temperatures (Figure ). Although M-NH 2 moieties have been obtained upon hydrogenation of terminal uranium or iridium nitride complexes, 56 the inaccessibility of oxidation states higher than +III for Lu would prevent the formation of a similar intermediate. As such, the formation of 6 is bound to proceed through direct hydrogenation of the N 2 -ligated complex 5, an unprecedented reactivity in RE chemistry. The observed N 2hydrogenation reactivity is all the more remarkable as it occurs on a doubly-reduced (N 2 ) 2-ligand. So far, the very rare examples of N 2 -functionalization on RE complexes (Figure ) have systematically required further activation of the N 2 ligand via formation of the (N 2 ) 3-radical anion. 11