| Summary: | Many Zintl phases take up hydrogen and form hydrides. Hydrogen atoms occupy interstitial sites formed by alkali or alkaline earth metals and/or bind covalently to the polyanions. The latter is the case for polyanionic hydrides like Sr<i>Tr</i><sub>2</sub>H<sub>2</sub> (<i>Tr</i> = Al, Ga) with slightly puckered honeycomb-like polyanions decorated with hydrogen atoms. This study addresses the hydrogenation behavior of <i>LnTr</i><sub>2</sub>, where the lanthanide metals <i>Ln</i> introduce one additional valence electron. Hydrogenation reactions were performed in autoclaves and followed by thermal analysis up to 5.0 MPa hydrogen gas pressure. Products were analyzed by powder X-ray and neutron diffraction, transmission electron microscopy, and NMR spectroscopy. Phases <i>Ln</i>Al<sub>2</sub> (<i>Ln</i> = La, Eu, Yb) decompose into binary hydrides and aluminium-rich intermetallics upon hydrogenation, while LaGa<sub>2</sub> forms a ternary hydride LaGa<sub>2</sub>H<sub>0.71(2)</sub>. Hydrogen atoms are statistically distributed over two kinds of trigonal-bipyramidal La<sub>3</sub>Ga<sub>2</sub> interstitials with 67% and 4% occupancy, respectively. Ga-H distances (2.4992(2) Å) are considerably longer than in polyanionic hydrides and not indicative of covalent bonding. <sup>2</sup>H solid-state NMR spectroscopy and theoretical calculations on Density Functional Theory (DFT) level confirm that LaGa<sub>2</sub>H<sub>0.7</sub> is a typical interstitial metallic hydride.
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