An intermediate phase LaMg2NiH4.6 found for the hydrogenation reaction of the ternary intermetallic compound LaMg2Ni to the complex transition-metal hydride LaMg2NiH7 has been investigated experimentally and theoretically. It is known that this reaction induces only minor rearrangement of the host metal atoms, and this feature is also observed for the intermediate compound. Another important feature is that the electronic structure of the intermediate phase is metallic, which is free from the energy barrier associated with the metal-insulator transition. The hydrogenation reaction to the intermediate phase proceeds even at room temperature, when the Ni-H clusters are formed. The internal bonding of the Ni-H clusters has a covalent nature, the same as those of NiH4 complexes in LaMg2NiH7. They most likely play as the precursor states for the following complex hydride formation. This picture is insightful to enhance the formation of complex hydrides.
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