  Following long-standing predictions associated with hydrogen,
high-temperature superconductivity has recently been observed in several
hydride-based materials. Nevertheless, these high-$T_c$ phases only exist at
extremely high pressures, and achieving high transition temperatures at ambient
pressure remains a major challenge. Recent predictions of the complex hydride
Mg$_{2}$IrH$_{6}$ may help overcome this challenge with calculations of
high-$T_c$ superconductivity (65 K$~<~T_c~<~$ 170 K) in a material that is
stable at atmospheric pressure. In this work, the synthesis of
Mg$_{2}$IrH$_{6}$ was targeted over a broad range of $P$-$T$ conditions, and
the resulting products were characterized using X-ray diffraction (XRD) and
vibrational spectroscopy, in concert with first-principles calculations. The
results indicate that the charge-balanced complex hydride Mg$_{2}$IrH$_{5}$ is
more stable over all conditions tested up to ca 28 GPa. The resulting hydride
is isostructural with the predicted superconducting Mg$_{2}$IrH$_{6}$ phase
except for a single hydrogen vacancy, which shows a favorable replacement
barrier upon insertion of hydrogen into the lattice. Bulk Mg$_{2}$IrH$_{5}$ is
readily accessible at mild $P$-$T$ conditions and may thus represent a
convenient platform to access superconducting Mg$_{2}$IrH$_{6}$ via
non-equilibrium processing methods.
