Hexammine Mg Borohydride: Structure & H2 Release

• Hexammine Magnesium Borohydride is a molecular solid with octahedral Mg coordination by NH₃ and tetrahedral BH₄⁻ units, underpinning its unique hydrogen storage properties.
• Its dynamic ammonia ligand disorder and cubic-to-ordered phase transition modulate ionic mobility and hydrogen release mechanisms.
• The material’s amphoteric hydrogen bonding between H⁻ in BH₄⁻ and H⁺ in NH₃ facilitates direct H₂ formation at lowered decomposition temperatures.

Hexammine magnesium borohydride is a molecular solid with the formula Mg(NH₃)₆(BH₄)₂, in which magnesium cations are octahedrally coordinated by six ammonia ligands and balanced by two tetrahydroborate anions. This compound, along with related magnesium borohydride ammoniates, is of significant interest for hydrogen storage applications due to its high hydrogen content and tunable decomposition properties. Recent crystallographic and spectroscopic studies have illuminated its structural dynamics, thermodynamic characteristics, and unique amphoteric hydrogen bonding environment—features that underpin its performance for hydrogen release and magnesium-ion conduction.

1. Crystal Structure and Orientational Disorder

At room temperature, hexammine magnesium borohydride crystallizes in a cubic structure that can be described as a K₂PtCl₆-type lattice (Nagle-Cocco et al., 5 Aug 2025). The Mg²⁺ ions are octahedrally coordinated by six NH₃ ligands, forming isolated Mg(NH₃)₆ octahedra. The BH₄⁻ anions occupy the faces of these octahedra, resulting in a network where each BH₄⁻ “points” toward Mg²⁺, yielding nearly linear B–H···Mg···H–B motifs.

Notably, synchrotron X-ray diffraction reveals that the NH₃ ligands are dynamically disordered at room temperature: nitrogen atoms of NH₃ reside on low-symmetry Wyckoff 96j sites, each with ¼ effective occupancy, indicative of both positional and orientational disorder. This implies that the ammonia ligands are rotationally mobile and distributed in multiple orientations within the crystal.

Upon cooling to 120 K, additional Bragg peaks emerge, consistent with a 2×2×2 expansion of the unit cell (from cubic Fm̅3m, a ≈ 10.76 Å at 300 K, V ≈ 1247.5 ų, to Fm̅3c, a ≈ 21.24 Å), marking a transition to an ordered phase where the ammonia orientation is “frozen out” and the Mg(NH₃)₆ octahedra tilt in a cooperative pattern.

Temperature	Space group	Lattice parameter (Å)	Structural order
Room temp	Fm̅3m	≈ 10.76	NH₃ orientationally disordered
120 K	Fm̅3c	≈ 21.24	NH₃ orientationally ordered

This structural transition, from dynamically disordered to locally ordered, is a defining property of the material and influences both its ionic mobility and hydrogen release characteristics.

2. Chemical Bonding and Amphoteric Hydrogen

The bonding environment in hexammine magnesium borohydride is characterized by the coexistence of both hydride-like (H⁻) and proton-like (H⁺) hydrogen atoms, a phenomenon described as amphoteric hydrogen behavior (Kiruthika et al., 2017, Varunaa et al., 2018). Specifically, each (BH₄)⁻ anion contains four hydrogens adjacent to boron in a relatively electron-rich (H⁻) state, while NH₃ contains three hydrogens adjacent to nitrogen in a more electron-deficient (H⁺) state.

Electronic structure analyses using density of states (DOS), charge density mapping, and electron localization functions (ELF) confirm this duality:

• The B–H bond features a predominantly ionic character with partial covalency, as indicated by significant charge localization between B and H.
• The N–H bond in NH₃ has a more iono-covalent character, with charge polarization toward nitrogen.
• Calculated Born effective charges (BEC) are noninteger, e.g., Z*{H(–1)} ~ –0.22 and Z*{H(+1)} ~ +0.32, and Bader charge analysis yields approximately ±0.55 e for H near B and N, respectively, instead of ±1.

This amphoteric nature permits close spatial proximity between oppositely charged hydrogen atoms, potentially leading to higher hydrogen packing densities and favorable storage capacities (Kiruthika et al., 2017, Varunaa et al., 2018).

3. Thermodynamics and Decomposition Pathways

Ammoniation of magnesium borohydride consistently results in thermodynamic destabilization relative to the non-ammoniated phase (Welchman et al., 2017). The formation energy of Mg(BH₄)₂ is more negative than that of its ammoniated analogs; thus,

ΔE_ammoniated > ΔE_plain (per atom),

which correlates with a reduced decomposition temperature. For example, the decomposition temperature of Mg(BH₄)₂ is ~260 °C, but drops to ~205 °C for Mg(BH₄)₂·2NH₃; similar or greater effects are expected in the hexammine case. This effect can be generalized as:

$$\text{Mg(BH}_4)_2\cdot y\,\text{NH}_3 \longrightarrow \text{Mg(BH}_4)_2\cdot (y-z)\,\text{NH}_3 + z\,\text{NH}_3$$

where ammonia release is facilitated by the lower stability of the ammoniated compound.

Magnesium’s intermediate Pauling electronegativity (χ_p ≈ 1.31) means that the Mg–NH₃ binding is not so strong as to kinetically lock NH₃, allowing moderate bond flexibility and enabling alternative dehydrogenation mechanisms. Ammoniation predominantly lowers T_dec via thermodynamic destabilization in magnesium systems rather than by imposing kinetic constraints.

4. Hydrogen Release Mechanisms

The dominant dehydrogenation pathway in Mg(NH₃)₆(BH₄)₂ is direct H₂ formation, enabled by dihydrogen bonding between hydride-like H^δ− (from BH₄⁻) and protic H^δ+ (from NH₃) (Welchman et al., 2017). Upon thermal activation or structural perturbation, H^δ− and H^δ+ approach sufficiently to facilitate the reaction:

$$\text{H}^{\delta+} + \text{H}^{\delta-} \longrightarrow \text{H}_2$$

The energetic barrier for this transformation is lower in mid-electronegativity systems like magnesium. NEB calculations show that the transition state for direct H₂ formation is accessible in these cases, with the ammonia ligands and BH₄ units forming dihydrogen bond networks that can “snap” to release H₂.

A schematic net dehydrogenation reaction may be written as:

$$\text{Mg(BH}_4)_2\cdot 6\,\text{NH}_3 \longrightarrow \text{Products} + \text{H}_2$$

where the “Products” may include rearranged phases with fewer NH₃ ligands. The avoidance of diborane production, a common side reaction in other borohydrides especially at higher χ_p, is another advantage.

5. Spectroscopic Signatures and Lattice Dynamics

Vibrational spectroscopy provides a means to assign and monitor the internal dynamics of the structure (Nagle-Cocco et al., 5 Aug 2025). Room-temperature IR measurements distinguish between several key vibrational modes:

• N–H symmetric stretch ν_s near 3265 cm⁻¹ and asymmetric stretch ν_a around 3350 cm⁻¹,
• N–H bending (δ) near 1196 cm⁻¹ (symmetric) and 1601 cm⁻¹ (degenerate),
• Multiple B–H stretching modes at 2153, 2236, and 2300 cm⁻¹ correlated with the BH₄⁻ subunit.

Splitting and shifting of B–H features, relative to precursor materials, arises from interactions within the K₂PtCl₆-type lattice framework and the dynamic coordination environment. The IR data additionally corroborate the model of orientational disorder at ambient temperature, showing broadened and complex lineshapes that resolve into sharper features when the structure is frozen into an ordered state below 120 K.

6. Implications for Hydrogen Storage and Ion Conduction

The material’s high hydrogen content—16.8 wt% hydrogen stored in both NH₃ and BH₄ units (Nagle-Cocco et al., 5 Aug 2025)—and the accessibility of the direct H₂ release pathway are favorable for reversible hydrogen storage. The coexistence of amphoteric hydrogen states enables higher volumetric densities than purely hydride-based counterparts due to attractive Coulomb interactions between H⁺ and H⁻ (Varunaa et al., 2018).

Moreover, the disorder of NH₃ ligands at room temperature may significantly enhance Mg²⁺ mobility, suggesting a possible utility as a Mg-ion conducting electrolyte in solid-state batteries. The transition to an ordered state at lower temperatures could reduce this mobility, highlighting the significance of dynamic ligand environments for practical applications.

7. Design Principles and Optimization Strategies

Optimal hydrogen storage performance can be realized by tuning both the degree of ammoniation and the structural arrangement of NH₃ and BH₄ ligands (Welchman et al., 2017):

• Controlled ammoniation (i.e., selection of NH₃ coordination number) allows for balancing T_dec and hydrogen purity by adjusting ΔE,
• Engineering the dihydrogen bond network, possibly by chemical substitution or process control, can further reduce the barrier for H₂ evolution,
• Structural tuning of the disorder–order transition may allow modulation of ion transport properties for dual-function hydrogen storage and battery electrolyte applications.

A plausible implication is that similar coordination environments and amphoteric hydrogen motifs in related borohydride ammoniates can be systematically exploited for material optimization across a family of compounds sharing these attributes.

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In conclusion, hexammine magnesium borohydride exemplifies how structural dynamics, amphoteric hydrogen chemistry, and tunable decomposition thermodynamics converge to yield a material system of interest for advanced hydrogen storage and magnesium-ion conductivity. Understanding and manipulating its orientational disorder, amphoteric bonding, and decomposition mechanisms are key to unlocking its full technological potential.