The ligand is the dihydrogen molecule H2
Organometallic chemistry - Hydrogen ligand and corresponding derivatives

A particularly interesting case is the attachment of the hydrogen molecule H₂ as such. This situation was discovered by Kubas.

Let's start by activating tungsten carbonyl through the action of cycloheptatriene, which replaces three molecules of CO :

The tungsten environment remains at 18 electrons. The action of a phosphine whose R groups are very bulky, such as the isopropyl group or the cyclohexyl group for example, leads to the substitution of cycloheptatriene C₇H₈ by only two phosphines precisely because of steric hindrance, thus bringing the metal's environment to 16 electrons :