The moderate synthesis conditions of $\mathrm{Ce}{\mathrm{H}}_9$ under cold compression demonstrate its remarkable chemical precompression effect, while the single occupied $4f$ electron configuration of Ce contributes to the rich chemical and physical properties observed in cerium-bearing compounds. In collaboration with machine-learning-accelerated crystal structure prediction, we predicted a thermodynamically stable superconducting phase $\mathrm{CeS}{\mathrm{c}}_2{\mathrm{H}}_{24}$ at 200 GPa. With considering more accurate quantum anharmonic effects, the thermodynamically stable pressure of $\mathrm{CeS}{\mathrm{c}}_2{\mathrm{H}}_{24}$ is further decreased to 117 GPa, at which pressure it exhibits a high superconducting critical temperature ($T_{\mathrm{c}}$) of ∼210 K. We found a different framework of bonding theory for hydrogen-based superconductors where strong metallic bonds in hydrides originate from near-free electronic states between the highest effective energy level and the Fermi level. Specifically, the delocalization of Ce-$4f$ orbitals in $\mathrm{CeS}{\mathrm{c}}_2{\mathrm{H}}_{24}$ provided a number of near-free electrons, thereby greatly enhancing metallic bonds and making the system energetically much more favorable. Our theory for $\mathrm{CeS}{\mathrm{c}}_2{\mathrm{H}}_{24}$ emphasizes the non-negligible role of metallic bonding in hydrides, which paves the way for developing high-temperature superconductors under experimentally accessible pressures.

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• Chemical bonding
• High-temperature superconductors
• Rare-earth alloys
• Superconductors