magnesium hydride#structure and bonding

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:{{cite journal|author1=Egon Wiberg |author2=Heinz Goeltzer |author3=Richard Bauer|year=1951|title=Synthese von Magnesiumhydrid aus den Elementen |trans-title=Synthesis of Magnesium Hydride from the Elements|lang=de|journal=Zeitschrift für Naturforschung B|volume=6b|page=394|url=http://zfn.mpdl.mpg.de/data/Reihe_B/6/ZNB-1951-6b-0394_n.pdf}}

:Mg + H₂ → MgH₂

Lower temperature production from Mg and H₂ using nanocrystalline Mg produced in ball mills has been investigated.{{cite journal | doi = 10.1016/S0925-8388(99)00073-0| title = Nanocrystalline magnesium for hydrogen storage| journal = Journal of Alloys and Compounds | volume = 288| issue = 1–2| pages = 217–225| year = 1999| last1 = Zaluska| first1 = A| last2 = Zaluski| first2 = L| last3 = Ström–Olsen| first3 = J.O}} Other preparations include:

• the hydrogenation of magnesium anthracene under mild conditions:{{cite journal | doi = 10.1002/anie.198008181| title = Catalytic Synthesis of Magnesium Hydride under Mild Conditions| journal = Angewandte Chemie International Edition in English| volume = 19| issue = 10| pages = 818| year = 1980| last1 = Bogdanović| first1 = Borislav| last2 = Liao| first2 = Shih-Tsien| last3 = Schwickardi| first3 = Manfred| last4 = Sikorsky| first4 = Peter| last5 = Spliethoff| first5 = Bernd}}

:Mg(anthracene) + H₂ → MgH₂

• the reaction of diethylmagnesium with lithium aluminium hydride{{cite journal | doi = 10.1021/ja01154a025| title = The Preparation of the Hydrides of Zinc, Cadmium, Beryllium, Magnesium and Lithium by the Use of Lithium Aluminum Hydride1| journal = Journal of the American Chemical Society| volume = 73| issue = 10| pages = 4585| year = 1951| last1 = Barbaras| first1 = Glenn D| last2 = Dillard| first2 = Clyde| last3 = Finholt| first3 = A. E| last4 = Wartik| first4 = Thomas| last5 = Wilzbach| first5 = K. E| last6 = Schlesinger| first6 = H. I}}
• 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.

The room temperature form α-MgH₂ has a rutile structure.{{cite journal | doi = 10.1107/S0365110X63000967 | title = Neutron diffraction study of magnesium deuteride | journal = Acta Crystallographica | volume = 16 | issue = 5 | pages = 352 | year = 1963 | last1 = Zachariasen | first1 = W. H | last2 = Holley | first2 = C. E | last3 = Stamper | first3 = J. F | doi-access = free }} There are at least four high pressure forms: γ-MgH₂ with α-PbO₂ structure,{{cite journal | doi = 10.1016/S0925-8388(99)00028-6| title = Structure of the high pressure phase γ-MgH2 by neutron powder diffraction| journal = Journal of Alloys and Compounds| volume = 287| issue = 1–2| pages = L4–L6| year = 1999| last1 = Bortz| first1 = M| last2 = Bertheville| first2 = B| last3 = Böttger| first3 = G| last4 = Yvon| first4 = K}} cubic β-MgH₂ with Pa-3 space group,{{cite journal | doi = 10.1103/PhysRevB.73.224102| title = Structural stability and pressure-induced phase transitions inMgH2| journal = Physical Review B| volume = 73| issue = 22| pages = 224102| year = 2006| last1 = Vajeeston| first1 = P| last2 = Ravindran| first2 = P| last3 = Hauback| first3 = B. C| last4 = Fjellvåg| first4 = H| last5 = Kjekshus| first5 = A| last6 = Furuseth| first6 = S| last7 = Hanfland| first7 = M| bibcode = 2006PhRvB..73v4102V}} orthorhombic HP1 with Pbc2₁ space group and orthorhombic HP2 with Pnma space group.{{cite journal | doi = 10.1143/JPSJ.75.074603| title = Structural Phase Transition of Rutile-Type MgH2at High Pressures| journal = Journal of the Physical Society of Japan| volume = 75| issue = 7| pages = 074603| year = 2006| last1 = Moriwaki| first1 = Toru| last2 = Akahama| first2 = Yuichi| last3 = Kawamura| first3 = Haruki| last4 = Nakano| first4 = Satoshi| last5 = Takemura| first5 = Kenichi| bibcode = 2006JPSJ...75g4603M}} Additionally a non stoichiometric MgH_(2-δ) has been characterised, but this appears to exist only for very small particles{{cite journal | doi = 10.1021/ja051508a| pmid = 16218629| title = Hydrogen Cycling of Niobium and Vanadium Catalyzed Nanostructured Magnesium| journal = Journal of the American Chemical Society| volume = 127| issue = 41| pages = 14348–54| year = 2005| last1 = Schimmel| first1 = H. Gijs| last2 = Huot| first2 = Jacques| last3 = Chapon| first3 = Laurent C| last4 = Tichelaar| first4 = Frans D| last5 = Mulder| first5 = Fokko M}}
(bulk MgH₂ is essentially stoichiometric, as it can only accommodate very low concentrations of H vacancies{{cite journal|last=Grau-Crespo|first=R.|author2=K. C. Smith |author3=T. S. Fisher |author4=N. H. de Leeuw |author5=U. V. Waghmare |title=Thermodynamics of hydrogen vacancies in MgH₂ from first-principles calculations and grand-canonical statistical mechanics|journal=Physical Review B|year=2009|volume=80|pages=174117|doi=10.1103/PhysRevB.80.174117|issue=17|arxiv=0910.4331|bibcode=2009PhRvB..80q4117G|s2cid=32342746}}).

The bonding in the rutile form is sometimes described as being partially covalent in nature rather than purely ionic;{{Cotton&Wilkinson6th}} 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.{{cite journal | doi = 10.1016/S0925-8388(03)00104-X| title = Charge density measurement in MgH2 by synchrotron X-ray diffraction| journal = Journal of Alloys and Compounds| volume = 356-357| pages = 84–86| year = 2003| last1 = Noritake| first1 = T| last2 = Towata| first2 = S| last3 = Aoki| first3 = M| last4 = Seno| first4 = Y| last5 = Hirose| first5 = Y| last6 = Nishibori| first6 = E| last7 = Takata| first7 = M| last8 = Sakata| first8 = M}}

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 10 K, formed following laser ablation of magnesium in the presence of hydrogen.{{cite journal | doi = 10.1021/jp046410h| title = Infrared Spectra of Magnesium Hydride Molecules, Complexes, and Solid Magnesium Dihydride| journal = The Journal of Physical Chemistry A| volume = 108| issue = 52| pages = 11511| year = 2004| last1 = Wang| first1 = Xuefeng| last2 = Andrews| first2 = Lester| bibcode = 2004JPCA..10811511W}} The Mg₂H₄ molecule has a bridged structure analogous to dimeric aluminium hydride, Al₂H₆.

MgH₂ readily reacts with water to form hydrogen gas:

:MgH₂ + 2 H₂O → 2 H₂ + Mg(OH)₂

At 287 °C it decomposes to produce H₂ at 1 bar pressure.{{cite book |title=Hydrogen and Energy |edition=illustrated |first1=T. R. |last1=McAuliffe |publisher=Springer |year=1980 |isbn=978-1-349-02635-7 |page=65 |url=https://books.google.com/books?id=71OuCwAAQBAJ}} [https://books.google.com/books?id=71OuCwAAQBAJ&pg=PA65 Extract of page 65] The high temperature required is seen as a limitation in the use of MgH₂ as a reversible hydrogen storage medium:{{cite journal | doi = 10.1038/35104634| pmid = 11713542| title = Hydrogen-storage materials for mobile applications| journal = Nature| volume = 414| issue = 6861| pages = 353–8| year = 2001| last1 = Schlapbach| first1 = Louis| last2 = Züttel| first2 = Andreas | url = http://doc.rero.ch/record/6025/files/zuttel_hsm.pdf| bibcode = 2001Natur.414..353S| s2cid = 3025203}}

:MgH₂ → Mg + H₂

{{Reflist|2}}

{{Magnesium compounds}}

Category:Magnesium compounds

Category:Metal hydrides