Copper–hydrides are known catalysts for several technologically important reactions such as hydrogenation of CO, hydroamination of alkenes and alkynes, and chemoselective hydrogenation of unsaturated ketones to unsaturated alcohols. Stabilizing copper-based particles by ligand chemistry to nanometer scale is an appealing route to make active catalysts with optimized material economy; however, it has been long believed that the ligand–metal interface, particularly if sulfur-containing thiols are used as stabilizing agent, may poison the catalyst. We report here a discovery of an ambient-stable thiolate-protected copper–hydride nanocluster [Cu25H10(SPhCl2)18]3– that readily catalyzes hydrogenation of ketones to alcohols in mild conditions. A full experimental and theoretical characterization of its atomic and electronic structure shows that the 10 hydrides are instrumental for the stability of the nanocluster and are in an active role being continuously consumed and replenished in the hydrogenation reaction. Density functional theory computations suggest, backed up by the experimental evidence, that the hydrogenation takes place only around a single site of the 10 hydride locations, rendering the [Cu25H10(SPhCl2)18]3– one of the first nanocatalysts whose structure and catalytic functions are characterized fully to atomic precision. Understanding of a working catalyst at the atomistic level helps to optimize its properties and provides fundamental insights into the controversial issue of how a stable, ligand-passivated, metal-containing nanocluster can be at the same time an active catalyst.

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An effective strategy is developed to synthesize high‐nuclearity Cu clusters, [Cu53(RCOO)10(C≡CtBu)20Cl2H18]+ (Cu53), which is the largest CuI/Cu0 cluster reported to date. Cu powder and Ph2SiH2 are employed as the reducing agents in the synthesis. As revealed by single‐crystal diffraction, Cu53 is arranged as a four‐concentric‐shell Cu3@Cu10Cl2@Cu20@Cu20 structure, possessing an atomic arrangement of concentric M12 icosahedral and M20 dodecahedral shells which popularly occurs in Au/Ag nanoclusters. Surprisingly, Cu53 can be dissolved in diethyl ether and spin coated to form uniform nanoclusters film on organolead halide perovskite. The cluster film can subsequently be converted into high‐quality CuI film via in situ iodination at room temperature. The as‐fabricated CuI film is an excellent hole‐transport layer for fabricating highly stable CuI‐based perovskite solar cells (PSCs) with 14.3 % of efficiency.