Transition metal hydrides are involved in many stoichiometric and catalytic organometallic reactions, such as hydrogenation. In these reactions the crucial step in the reaction path is the co-ordination of an intact H2 molecule to the metal centre and the ligand-dynamics of the resulting coordination compound. We propose to carry out an accurate single crystal neutron diffraction study on the octahedral complexes trans-[(P iPr3)]2IrI(H)2(H2) to investigate the temperature dependence of the ADP’s. Our aim is to obtain insight on the relationships between the large amplitude motion of the H atoms, the exchange mechanism between H and H2 and the rapid reversible absorption/desorption equilibrium.