The calculated binding free energies show that the preferential adsorption of N2 over H2 at the [RuH6]* active center is further enhanced at lower temperatures (Figure ). This feature is critical for the understanding of the catalytic mechanism, particularly when combined with the kinetics of dissociative hydrogen chemisorption and hydrogen transfer over the [RuH6] centers. Figure displays the free energy landscape for dissociation of chemisorbed H2 (state-I) on the [RuH6] active center (eqn.( )). Although the dissociative chemisorption of H2 into two lattice Hs is facile, the subsequent transfer of lattice H, as [RuH7] (state-II), via a transition state (TSI-II), is prohibited by a high activation energy of 1.1 eV (Figure ) as estimated with nudged elastic band simulations. The transfer of the chemisorbed H2 to lattice H is Li-mediated and encounters repulsive force from neighboring Hs, making the activation barrier high. Instead, at low temperature, the active center retains the hydrogen atoms to form a [RuH6]H2 complex (state-I), which is not a very stable state and can easily desorb H2 at temperatures above 300 K to release the [RuH6] catalytic site for N2 activation (see Figure ). The mechanism of H2 chemisorption/desorption ensures not all [RuH6] centers are converted to [RuH7] complexes, even in an H2-rich environment. Thus, the catalyst surface is not hydrogen over-saturated by lattice H that bonds firmly to the respective Ru blocking the active sites. This contrasts with Ru and other late transition metal catalysts, where H-poisoning, due to favorable thermodynamics effectively prevents N2 adsorption. The opposing behavior between Ru metal and Li4RuH6 catalysts is elegantly captured in the NH3 formation rate under varying pressure (Figure ). The lack of hydrogen poisoning effect allows enhanced ammonia production at a higher hydrogen partial pressure on Li4RuH6.