Iron hydride

A few molecular compounds have been detected in extreme environments (such as stellar atmospheres) or in small amounts at very low temperatures.

The two elements form a metallic alloy above 35,000 standard atmospheres (3.5 GPa) of pressure, that has been advanced as a possible explanation for the low density of Earth's "iron" core.

Small amounts of hydrogen (up to about 0.08% by weight) are absorbed into iron as it solidifies from its molten state.

Despite the fleeting nature of binary iron hydrides, there are many fairly stable complexes containing iron-hydrogen bonds (and other elements).

Methanogens, archaea, bacteria and some unicellular eukaryotes contain hydrogenase enzymes that catalyse metabolic reactions involving free hydrogen, whose active site is an iron atom with Fe–H bonds as well as other ligands.