The low-pressure stabilization of superconducting hydrides with high critical temperatures $\left(T_c\mathrm{s}\right)$ remains a significant challenge, and experimentally verified superconducting hydrides are generally constrained to a limited number of structural prototypes. Ternary transition metal complex hydrides (hydrido complexes)—typically regarded as hydrogen storage materials—exhibit a large range of compounds stabilized at low pressure with recent predictions for high-$T_c$ superconductivity. Motivated by this class of materials, we investigated complex hydride formation in the Mg–Pt–H system, which has no known ternary hydride compounds. Guided by ab initio structural predictions, we successfully synthesized a novel complex transition metal hydride, ${\mathrm{Mg}}_4{\mathrm{Pt}}_3{\mathrm{H}}_6$, using laser-heated diamond anvil cells. The compound forms in a body-centered cubic structural prototype at moderate pressures between $\sim 8\text{and}25\mathrm{GPa}$. Unlike the majority of known hydrido complexes, ${\mathrm{Mg}}_4{\mathrm{Pt}}_3{\mathrm{H}}_6$ is metallic, with formal charge described as $4{\left[\mathrm{Mg}\right]}^{2+}·3{\left[{\mathrm{PtH}}_2\right]}^{2-}$. X-ray diffraction measurements obtained during decompression reveal that ${\mathrm{Mg}}_4{\mathrm{Pt}}_3{\mathrm{H}}_6$ remains stable upon quenching to ambient conditions. Magnetic-field and temperature-dependent electrical transport measurements indicate ambient-pressure superconductivity with $T_c$ $(50\%)=2.9\mathrm{K}$, in reasonable agreement with theoretical calculations. These findings clarify the phase behavior in the Mg–Pt–H system, highlight important synergies between computational/experimental approaches, and provide valuable insights for transition metal complex hydrides as a new class of hydrogen-rich superconductors.

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• Electronic structure
• Pressure effects
• Structural properties
• Superconductivity
• Synthesis
• Hydrides