Hydrogen has a tendency to react with reactive metals like lithium, sodium to give corresponding hydrides
2Li+H2 → 2LiH
2Na+H2 → 2NaH
These hydrides are used as reducing agents in synthetic organic chemistry. It is also used to prepare important hydrides such as lithium aluminium hydride and sodium boro hydride (organic reducing agents).
4LiH + AlCl3 → Li[AIH4] + 3LiCl
4NaH + B(OCH3)3 → Na[BH4] + 3CH3ONa