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.
