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

The X₂MH₆ family, consisting of an electropositive cation Xⁿ⁺ 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, Mg₂IrH₆ and Ca₂IrH₆, showing why the former may possess superconducting properties rivaling those of the cuprates, whereas the latter does not. Within Mg₂IrH₆ the vibrations of the anions IrH₆ ^4- anions are key for the superconducting mechanism, and they induce coupling in the $$ set of orbitals, which are antibonding between the H 1s and the Ir $$ 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.