Magnesium iron hexahydride explained

Magnesium iron hexahydride is an inorganic compound with the formula Mg₂FeH₆. It is a green diamagnetic solid that is stable in dry air. The material is prepared by heating a mixture of powdered magnesium and iron under high pressures of hydrogen:^[1]

2 Mg + Fe + 3 H₂ → Mg₂FeH₆

Structure

The compound is isomorphous with K₂PtCl₆, i.e., their connectivities and structures are the same. The [FeH₆]⁴⁻ centre adopts octahedral molecular geometry with Fe-H distances of 1.56 Å. The Mg²⁺ centres are bound to the faces of the octahedron, with Mg-H distances of 2.38 Å. Several related compounds are known including salts of [RuH₆]⁴⁻, [OsH₆]⁴⁻, and [PtH₆]²⁻ anions.^[2]

Soluble derivatives

Although Mg₂FeH₆ is not soluble in ordinary solvents, related derivatives are. For example, the related salt Mg₄Br₄(THF)₄FeH₆ is soluble as are related alkoxides. Measurements on such compounds suggest that the hydride ligand exerts a weaker crystal field than cyanide.^[3]

Notes and References

1. J. J. Didisheim, P. Zolliker, K. Yvon, P. Fischer, J. Schefer, M. Gubelmann, A. F. Williams "Dimagnesium iron(II) hydride, Mg₂FeH₆, containing octahedral FeH₆⁴⁻ anions" Inorganic Chemistry 1984, vol. 23, pp 1953–1957. .
2. Robert Bau, Mary H. Drabnis "Structures of transition metal hydrides determined by neutron diffraction" Inorganica Chimica Acta 1997, vol. 259, 27ff.
3. R.B. King "Structure and bonding in homoleptic transition metal hydride anions" Coordination Chemistry Reviews, 2000, vol. 200–202, p. 813–829.