We used density-functional calculations to clarify the origin of the unusual sevenfold coordination of Ru by H in Na3RuH7. We found that the D5h symmetry of the ligands enables the formation of strong covalent bonds of Ru and H through ligand-field effects, stabilizing the sevenfold coordination. We also examined the possible synthesis of the hypothetical 3d analog, Li3FeH7, which has a gravimetric hydrogen density of 8.4 mass%. The calculated enthalpy change of -16 kJ/mol H2 for the reaction, 3 LiH + Fe + 2 H2 → Li3 FeH7, reveals a possible route to a stable complex hydride containing [FeH7]3-.
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