DFT calculations were performed to shed light on the mechanism of the unusual H2 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 H2 in between two molecules of 2 (1.2 kcal•mol -1 ). This adduct yields to the side-on coordination of H2 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 H2 (see Figure ). This coordination has thus decreased the energy of the H2 σ* orbital (which is now involved in the SOMO), allowing further reduction of H2 via a bimetallic transition state (TS). At the TS, H2 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.