Heating a solution of 2 in C6D6 at 80 °C for several days led to an unexpectedly clean and welldefined 1 H NMR spectrum, showing the formation of the Lu III hydride complex 3-H and the cyclometallated complex 4 (Figure ) in close to equimolar amounts (see Figures ). The identity of 4 was testified by its reactivity towards hydrogenolysis, as complete and immediate conversion into 3-H occurred upon addition of H2 (Figure ). A similar hydrogenolysis reactivity had been earlier reported by Andersen and co-workers on the cerium analogue of 4. The 1 H NMR spectrum of 4 displays, notably, two doublets at  0.65 and -0.18 ppm, with a 2 JHH coupling constant of 13.5 Hz, for the diastereotopic protons of the cyclometallated methylene group. The characterization data of 3-H will be discussed in detail in the next section. The hydrogenolysis reaction is reversible as heating a degassed solution of 3-H under static vacuum for several days led to complete conversion back to 4 (Figures S14-15). The formation of 4 and its reactivity with H2 to form 3-H were also investigated computationally. The formation of 4 implies a bimetallic transition state (TS) on the triplet spin potential energy surface (PES) where the hydrogen from one t Bu group of one molecule of 2 is transferred to the Lu center of a second molecule of 2 (Figure ). The associated barrier is 39.0 kcal•mol -1 , in line with the relative stability of 2 at room temperature and the experimental observation of a slow thermal degradation at 80 °C. Yet, the reaction is exothermic by 20.8 kcal•mol -1 due to the formation of one equivalent of 4 and one of 3-H. The hydrogenolysis of complex 4 to form 3-H (Figure ) is both kinetically (barrier of 7.2 kcal•mol -1 ) and thermodynamically favorable (-15.8 kcal•mol -1 ). The activation barrier of 22.9 kcal•mol -1 for the reverse transformation is fully consistent with the dehydrogenation of 3-H into 4 accessible upon heating.