[en] Highlights: •The systematic study of its structure and electronic structure in gas and solid phase. •The gas phase structures indicate that Li⁺ cation and Na⁺ cation acts as very different roles in the dehydrogenation. •Potential energy curves of dehydrogenation were obtained by CCSD(T) method. •Obtained the dehydrogenation rates similar to the experimental values. -- Abstract: This study is based on the synthesis of a mixed metal amidoborane, Na[Li(NH₂BH₃)]₂ (SLAB), the first example of an inorganic sodium–lithium compound. This paper is the first systematic study of its structure and dehydrogenation mechanism, and the results obtained were consistent with the experimental results. The first principle method was used to study the structure of SLAB in solid phase, while the second Moller–Plesset Perturbation Theory was used for gas-phase kinetic studies. Potential energy curves were obtained by CCSD(T) method. Three mechanistic pathways were designed to study its dehydrogenation, which include A pathway (without the cleavage of N–B bond (S and S′ pathways)), B pathway (with the cleavage of the intramolecular N–B bond before dehydrogenation), and D pathway (with the formation of direct dihydrogen bonds). Na⁺ cation movement was proved to play a very important role in the hydrogen-transfer process. Finally, a possible dissociation pathway (A pathway) was confirmed and dehydrogenation rates similar to the experimental values were obtained