Why Mg₂IrH₆ Is Predicted to Be a High‐Temperature Superconductor, But Ca₂IrH₆ Is Not

Abstract The X 2 MH 6 family, consisting of an electropositive cation X n+ and a main group metal M octahedrally coordinated by hydrogen, have been identified as promising templates for high‐temperature conventional superconductivity. Herein, we analyze the electronic structure of two members of this family, Mg 2 IrH 6 and Ca 2 IrH 6 , showing why the former may possess superconducting properties rivaling those of the cuprates, whereas the latter does not. Within Mg 2 IrH 6 the vibrations of the anions IrH 6 4− anions are key for the superconducting mechanism, and they induce coupling in the set of orbitals, which are antibonding between the H 1 s and the Ir or orbitals. Because calcium possesses low‐lying d ‐orbitals, →Ca d back‐donation is preferred, quenching the superconductivity. Our analysis explains why high critical temperatures were only predicted for second or third row X metal atoms, and may provide rules for identifying likely high‐temperature superconductors in other systems where the antibonding anionic states are filled.