Iron hydride


An iron hydride is a chemical system which contains iron and hydrogen in some associated form.
Because of the common occurrence of those two elements in the universe, possible compounds of hydrogen and iron have attracted attention. A few molecular compounds have been detected in extreme environments or detected in small amounts at very low temperatures. The two elements form a metallic alloy above 35000 atmospheres of pressure, that has been advanced as a possible explanation for the low density of Earth's "iron" core. However those compounds are unstable when brought to ambient conditions, and eventually decompose into the separate elements.
Small amounts of hydrogen are absorbed into iron as it solidifies from molten state. Although the H2 is simply an impurity, its presence can affect the mechanical properties of the material.
Despite the fleeting nature of binary iron hydrides, there are many fairly stable complexes containing iron-hydrogen bonds.

Overview

Solid solutions

Iron and iron-based alloys can form solid solutions with hydrogen, which under extreme pressure may reach stoichiometric proportions, remaining stable even at high temperatures and that is reported to survive for a while under ambient pressure, at temperatures below 150K.

Binary compounds

Molecular compounds

Complexes displaying iron–hydrogen bonds include, for example:
Complexes are also known with molecular hydrogen ligands.

Biological occurrence

, 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.