Magnesium iron hexahydride is an inorganic compound with the formula Mg<sub>2</sub>FeH<sub>6</sub>. 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:
The compound is isomorphous with K<sub>2</sub>PtCl<sub>6</sub>, i.e., their connectivities and structures are the same. The [FeH<sub>6</sub>]<sup>4âÂÂ</sup> centre adopts octahedral molecular geometry with Fe-H distances of 1.56 à. The Mg<sup>2+</sup> centres are bound to the faces of the octahedron, with Mg-H distances of 2.38 à. Several related compounds are known including salts of [RuH<sub>6</sub>]<sup>4âÂÂ</sup>, [OsH<sub>6</sub>]<sup>4âÂÂ</sup>, and [PtH<sub>6</sub>]<sup>2âÂÂ</sup> anions.
Although Mg<sub>2</sub>FeH<sub>6</sub> is not soluble in ordinary solvents, related derivatives are. For example, the related salt Mg<sub>4</sub>Br<sub>4</sub>(THF)<sub>4</sub>FeH<sub>6</sub> is soluble as are related alkoxides. Measurements on such compounds suggest that the hydride ligand exerts a weaker crystal field than cyanide.