After various attempts, we present a new alkaline-earth derivative of hydrazine borane (N2H4BH3, HB), magnesium hydrazinidoborane (Mg(N2H3BH3)2, Mg(HB)2, 10.5 wt % H), that was undoubtedly identified by FTIR and 11B MAS NMR spectroscopy. Mg(HB)2 was obtained by an alternative synthesis route, which is the reaction between HB and di-n-butylmagnesium in THF. The dehydrogenation properties of this compound were evaluated by two different approaches: an “open” system by thermogravimetric analysis and differential scanning calorimetry, and in a closed system, by heating the compound under isothermal conditions. Different results were obtained depending on the approach. Unlike other boron- and nitrogen-based compounds, it is likely that when Mg(HB)2 is heated in a closed system, the dehydrogenation is limited and it occurs mainly due to the homopolar interaction between the protic hydrogen atoms of the molecule. Also, Mg(HB)2 presents a contrasting thermal behavior in comparison with previous HB derivatives. In addition, a characterization by X-ray photoelectron spectroscopy was performed, and we detected instability of Mg(HB)2 when it was irradiated with the X-ray beam. All of these results are presented and discussed in the context of materials for hydrogen storage in the solid-state.