Magnesium hydride

Magnesium hydride
IUPAC name Magnesium hydride
Identifiers
CAS number	7693-27-8 Y
PubChem	107663
ChemSpider	16787263 Y
ChEBI	CHEBI:25107 Y
Jmol-3D images	Image 1
SMILES [MgH2]
InChI InChI=1S/Mg.2H Y Key: RSHAOIXHUHAZPM-UHFFFAOYSA-N Y InChI=1/Mg.2H/rH2Mg/h1H2 Key: RSHAOIXHUHAZPM-HZAFDXBCAG
Properties
Molecular formula	MgH2
Molar mass	26.3209 g/mol
Appearance	white crystals
Density	1.45 g/cm3
Melting point	327 °C
Solubility in water	decomposes
Hazards
EU Index	Not listed
Main hazards	pyrophoric[1]
Related compounds
Other cations	Beryllium hydride Calcium hydride Strontium hydride Barium hydride
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Magnesium hydride is the chemical compound MgH₂. It contains 7.66% by weight of hydrogen and has been studied as a potential hydrogen storage medium.^[2]

Preparation

In 1951 preparation from the elements was first reported involving direct hydrogenation of Mg metal at high pressure and temperature (200 atmospheres, 500°C) with MgI₂ catalyst:^[3]

Mg + H₂ → MgH₂

Lower temperature production from Mg and H₂ using nano crystalline Mg produced in ball mills has been investigated.^[4] Other preparations include:

• the hydrogenation of magnesium anthracene under mild conditions:^[5]

Mg(anthracene) + H₂ → MgH₂

• the reaction of diethylmagnesium with lithium aluminium hydride^[6]

• product of complexed MgH₂ e.g. MgH₂.THF by the reaction of phenylsilane and dibutyl magnesium in ether or hydrocarbon solvents in the presence of THF or TMEDA as ligand. ^[1]

Structure and bonding

The room temperature form β-MgH₂ has a rutile structure.^[7] There are two high pressure forms, α-MgH₂ with α-PbO₂ structure^[8] and γ-MgH₂.^[9] Additionally a non stoichiometric MgH_(2-δ) has been characterised, but this appears to exist only for very small particles^[10]
(bulk MgH₂ is essentially stoichiometric, as it can only accommodate very low concentrations of H vacancies^[11]).

The bonding in the rutile form is sometimes described as being covalent in nature rather than purely ionic;^[12] charge density determination by synchrotron x-ray diffraction indicates that the magnesium atom is fully ionised and spherical in shape and the hydride ion is elongated.^[13] Molecular forms of magnesium hydride, MgH, MgH₂, Mg₂H, Mg₂H₂, Mg₂H₃, and Mg₂H₄ molecules identified by their vibrational spectra have been found in matrix isolated samples at below 10K, formed following laser ablation of magnesium in the presence of hydrogen.^[14] The Mg₂H₄ molecule has a bridged structure analogous to dimeric aluminium hydride, Al₂H₆.^[14]

Reactions

MgH₂ readily reacts with water to form hydrogen gas: MgH₂ + 2H₂O → 2H₂ + Mg(OH)₂

At 300°C decomposes to produce H₂ at 1 bar pressure, the high temperature required is seen as a limitation in the use of MgH₂ as a reversible hydrogen storage medium:^[15]

MgH₂→ Mg+ H₂

Potential use for hydrogen storage

Its potential as a reversible "storage" medium for hydrogen has led to interest in improving the hydrogenation and dehydrogenation reaction kinetics.^[15][16] An alternative approach under investigation is the production of a p umpable slurry of MgH₂ which is safe to handle and releases H₂ by reaction with water, with reprocessing of the Mg(OH)₂ into MgH₂.[2] An application (yet to be examined) for a US Patent (US 2010/0163434 A1) [3] has been made in respect of a hydrogen energy storage system using laser excitation to assist adsorption of hydrogen gas from magnesium hydride.

References

1. ^ ^{a b} Synthesis of magnesium hydride by the reaction of phenylsilane and dibutylmagnesium , Michalczyk M.J., Organometallics; (1992); 11(6); 2307-2309. doi:10.1021/om00042a055
2. ^ Catalytic Synthesis of Organolithium and Organomagnesium Compounds and of Lithium and Magnesium Hydrides - Applications in Organic Synthesis and Hydrogen Storage, Bogdanovic B., Angewandte Chemie International Edition in English, 24, 4, 262–73, doi:10.1002/anie.198502621
3. ^ Wiberg, Goeltzer, Bauer,1951, Z naturforsch 6b, 394, (1951)
4. ^ Nanocrystalline magnesium for hydrogen storage, A Zaluska, L Zaluski, JO Ström–Olsen, Journal of Alloys and Compounds, 288, 1-2, 1999, 217-225, doi:10.1016/S0925-8388(99)00073-0
5. ^ Catalytic Synthesis of Magnesium Hydride under Mild Conditions, Bogdanovic B., Liao S-T, Schwickardi M, Sikorsky P., Spliethoff B., Angewandte Chemie International Edition in English, 19,(1980), 10, 818 – 819, doi:10.1002/anie.198008181
6. ^ The Preparation of the Hydrides of Zinc, Cadmium, Beryllium, Magnesium and Lithium by the Use of Lithium Aluminum Hydride, Barbaras G.D., Dillard C., Finholt A. E., Wartik T, Wilzbach K. E., Schlesinger H. I., J. Am. Chem. Soc.; 1951; 73(10); 4585-4590, doi:10.1021/ja01154a025
7. ^ Neutron diffraction study of magnesium deuteride, Zachariasen W.H., Holley C.E, Stamper J.F. Jnr, Acta Cryst. (1963) 16, 352-353, doi:10.1107/S0365110X63000967
8. ^ Michael D. Hampton, Dmitry V. Schur, Svetlana Yu. Zaginaichenko, V. I. Trefilov, Hydrogen Materials Science and Chemistry of Metal Hydrides: Proceedings of the NATO Advanced Research Workshop, Katsiveli, Yalta, Ukraine, 2002, Springer, ISBN 1402007302
9. ^ Structure of the high pressure phase γ-MgH₂ by neutron powder diffraction, Bortz M., Bertheville B., Böttger G., Yvon K., Journal of Alloys and Compounds, 287, 1-2, (1999), L4-L6, doi:10.1016/S0925-8388(99)00028-6
10. ^ Hydrogen Cycling of Niobium and Vanadium Catalyzed Nanostructured Magnesium,Schimmel, H. G.; Huot, J.; Chapon, L. C.; Tichelaar, F. D.; Mulder, F. M.,J. Am. Chem. Soc.; (Article); 2005; 127(41); 14348-14354. doi:10.1021/ja051508a
11. ^ Grau-Crespo, R.; K. C. Smith, T. S. Fisher, N. H. de Leeuw, and U. V. Waghmare (2009). "Thermodynamics of hydrogen vacancies in MgH₂ from first-principles calculations and grand-canonical statistical mechanics". Physical Review B 80: 174117. http://prb.aps.org/abstract/PRB/v80/i17/e174117. 
12. ^ Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, ISBN 0-471-19957-5 
13. ^ Charge density measurement in MgH₂ by synchrotron X-ray diffraction, T. Noritake, S. Towata, M. Aoki, Y. Seno, Y. Hirose, E. Nishibori, M. Takata and M. Sakata, Journal of Alloys and Compounds, 356-357, (2003), 84-86, doi:10.1016/S0925-8388(03)00104-X
14. ^ ^{a b} Infrared Spectra of Magnesium Hydride Molecules, Complexes, and Solid Magnesium Dihydride, Xuefeng Wang and Lester Andrews, J. Phys. Chem. A, 108 (52), 11511 -11520, 2004. doi:10.1021/jp046410h
15. ^ ^{a b} Hydrogen-storage materials for mobile applications, L. Schlapbach and A. Züttel, Nature 414, 353 (2001).doi:10.1038/35104634[1]
16. ^ J Huot Hydrogen in Metals (2002) in New Trends in Intercalation Compounds for Energy Storage, Chrisitan Julien, J. P. Pereira-Ramos, A. Momchilov, Springer, ISBN 1402005946