The first hydride-containing 2-electron palladium/copper alloys, [PdHCu₁₁ {S₂ P(Oⁱ Pr)₂ }₆ (C≡CPh)₄ ] (PdHCu₁₁ ) and [PdHCu₁₂ {S₂ P(Oⁱ Pr)₂ }₅ {S₂ PO(Oⁱ Pr)} (C≡CPh)₄ ] (PdHCu₁₂ ), are synthesized from the reaction of [PdH₂ Cu₁₄ {S₂ P(Oⁱ Pr)₂ }₆ (C≡CPh)₆ ] (PdH₂ Cu₁₄ ) with trifluoroacetic acid (TFA). X-ray diffraction reveals that the PdHCu₁₁ and PdHCu₁₂ kernels consist of a central PdH unit encapsulated within a vertex-missing Cu₁₁ cuboctahedron and complete Cu₁₂ cuboctahedron, respectively. DFT calculations indicate that both PdHCu₁₁ and PdHCu₁₂ can be considered as axially-distorted 2-electron superatoms. PdHCu₁₁ shows excellent HER activity, unprecedented within metal nanoclusters, with an onset potential of -0.05 V (at 10 mA cm^-2 ), a Tafel slope of 40 mV dec^-1 , and consistent HER activity during 1000 cycles in 0.5 M H₂ SO₄ . Our study suggests that the accessible central Pd site is the key to HER activity and may provide guidelines for correlating catalyst structures and HER activity.

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