Protons complex with hydride ions to form molecular hydrogen, H2, a uniquely simple and much studied diatomic molecule.
The H+ + H– H2 reaction is not reversible: H2 does not act as a proton donor (although at high temperature, when exposed to high energy UV radiation or when absorbed onto a metallic surface, H2 can homolytically dissociate: radical cleavage).
As protons and hydride ions do not exist as independent species, they require "delivery" by donor complexes, ie reagents. Protons, H+ ions, are supplied by Brønsted acids and hydride ions by hydride donor complexes.
For example, hydrogen chloride an H+ donor reacts with sodium hydride an H– donor to give diatomic hydrogen and sodium chloride:
HCl + NaH H2 + NaCl
[H3]+, the product of H+ and H2, is the simplest possible triatomic molecular ion – it has only two electrons – and is of considerable theoretical interest.
The [H3]+ molecular ion occupies an important position in theoretical models of interstellar chemistry as the [H3]+ forms in hydrogen-rich interstellar gas clouds. The [H3]+ ion can protonate carbon, oxygen and other atoms, thereby initiating the interstellar synthesis of nearly 100 molecules including: hydroxyl radicals (HO•), carbon monoxide, ethanol, linear polyacetylenes and cyclopropenylidene.
(In this author's opinion the [H3]+ ion should be called the 'hydronium ion', and [OH3]+ should be the 'oxonium ion'.)