We synthesized Na₂[B₁₂H₁₂] by a solid state procedure and thermal decomposition of Na[B₃H₈], and calculated from a first-principles approach the thermodynamic and structural properties. In particular, the calculated enthalpy of formation of the monoclinic structure, at T = 0 K, of −1086.196 kJ mol⁻¹ showed that it is a very stable compound. Therefore, in case it were formed during the thermal decomposition of NaBH₄, it would be rather considered a product, which, in addition, prevents the subsequent re-hydrogenation process because of its low reactivity to hydrogen. We reported the isotherms of absorption of H₂, O₂, and H₂O, calculated both theoretically and experimentally.