The deuterium atoms in both Ce 2Ni 7D ∼4 and CeNi 3D 2.8 occupy interstices in the CeNi 2 structural slabs only, while the interstices in the bulk of the CeNi 5 slabs remain empty. A considerable fraction of these deuterium atoms surround nickel in deformed tetrahedral configurations reminiscent of the [NiH 4] 4− complexes found in complex metal hydrides, like Mg 2NiH 4 and LaMg 2NiH 7. The hydrogen equilibrium pressures of both systems are similar (0.1–0.2 bar at 50 °C). These findings not only provide further evidence for directional bonding effects in hydrides that are traditionally considered as “interstitial”, but also suggest that the thermal stability of metal hydrides having composite crystal structures is correlated with metal-hydrogen bond formation/breaking in specific structural units.