Why Mg₂IrH₆ is predicted to be a high temperature superconductor, but Ca₂IrH₆ is not

The X₂MH₆ family, consisting of an electropositive cation X and a main group metal M octahedrally coordinated by hydrogen, has been predicted to hold promise 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 IrH₆⁴⁻ anions are key for the superconducting mechanism, and they induce coupling in the eg^* set, which are antibonding between the H 1s and the Ir dx²-y² or dz² orbitals. Because calcium possesses low-lying d-orbitals, eg^* → 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 hold implications for superconductivity in other systems where the antibonding anionic states are filled.