To the best of our knowledge, direct H 2 splitting by molecular lanthanide complexes is unprecedented to date. This strategy offers a novel route for the synthesis of molecular Ln III -H species, typically obtained by hydrogenolysis of Ln-alkyl bonds. It can be noted that, in rare instances, molecular RE hydrides have been obtained upon treatment of trivalent complexes with strong alkali metal reductants. However, such reactions did not involve H 2 activation but rather undefined hydrogen abstraction from solvents or possible aromatic C-H activation. Although elemental zerovalent lanthanides are known to react with hydrogen at elevated temperatures to afford binary Ln hydrides, no such reaction has been observed with molecular Ln species. Weak coordination of H 2 to a lanthanide complex has only been spectroscopically evidenced for the base-free Eu II complex [Eu(C 5 Me 5 ) 2 ], in which case no metal oxidation and formation of a hydride complex occurred. Very recently, this phenomenon has been extended to 5f elements with spectroscopic and computational evidence of the reversible formation of a U III -H 2 complex. Regarding heterogeneous lanthanide systems, nondissociative dihydrogen binding has also been very recently evidenced by solid-state NMR studies under pressure of H 2 . Finally, it is worth noting that, within the actinide series, some U II and Th II complexes were reported to activate H 2 , leading to higher-valent hydride complexes, but the corresponding mechanisms remain elusive. DFT calculations were performed to shed light on the mechanism of the unusual H 2 activation reactivity of 2. Reaction profiles were calculated on the triplet and singlet Potential Energy Surface (PES) (Figure ), and the most probable profile is discussed (Figure ). The reaction begins with the formation of a weak van der Waals adduct of H 2 in between two molecules of 2 (1.2 kcal•mol -1 ). This adduct yields to the side-on coordination of H 2 to one Lu II center while the second Lu II remains in the vicinity (+5.4 kcal•mol -1 ). It is important to note that these two adducts do not imply oxidation of the Lu center since the most stable one is on the triplet PES and the unpaired spin density is mainly located at the two Lu centers (see Figure ). However, the H-H distance (0.82 Å) is elongated, and the SOMO implies interaction between a d orbital on Lu II and the σ* of H 2 (see Figure ). This coordination has thus decreased the energy of the H 2 σ* orbital (which is now involved in the SOMO), allowing further reduction of H 2 via a bimetallic transition state (TS). At the TS, H 2 is undergoing a two-electron reduction that allows the disruption of the H-H bond with an accessible barrier of 24.2 kcal•mol -1 . The value for this activation barrier is fully consistent with the relatively slow reaction observed at room temperature.