LnNiSi (Ln = La-Nd) comprising a three-dimensional NiSi framework has electrons in the crystallographic cavity space. In the temperature region 473-773 K, it accepts the insertion/de-insertion of hydrogen topotactically without a change in unitcell volume. The insertion of hydrogens into the cavity space is accompanied by a redox reaction with the orbitals of atoms constituting the cavity wall. Having small work functions, such intermetallic electrides exhibit metallic electrical and magnetic properties. Owing to a high electron-donating power and reversible exchange between hydrogen and the electrons, Ru5wt%-loaded LaNiSi powder worked as an efficient catalyst for ammonia synthesis under ambient pressure.