Owing to advantages in synthesis, separation, structure determination, and low cost (compared to noble metal nanoclusters), Ag/Cu hydride clusters (and their alloys) have received increasing research interest in recent decades and have shown great potential in mediating reduction reactions and H₂ storage applications. The atomic precision of the Ag/Cu hydride clusters with the combination of single-crystal X-ray diffraction, ^1/2H nuclear magnetic resonance, electrospray ionization mass spectrometry, density functional theory, and particularly, single-crystal neutron diffraction, has provided pivotal information regarding its structural characteristics, facilitating a deep understanding of the inherent bonding principles therein. This review summarizes the research progress of atomically precise Ag/Cu hydride clusters (and their alloys) over the past three years (2021–2023), mainly focusing on the synthesis, structure analysis, and catalytic applications of the hydride clusters. We believe that this review can benefit the future design of different types of metal hydride clusters and aid in their application in various redox reactions.

The redox property of the ultrasmall coinage nanoclusters (with several to tens of Au/Ag atoms) has elucidated the electron-transfer capacity of nanoclusters, and has been successfully utilized in a variety of redox conversions (such as from CO₂ to CO). Nevertheless, their biological applications are mainly restricted by the scarcity of atomically precise, water-soluble metal nanoclusters, and the limited application (mainly on the decomposition of H₂O₂ in these days). Herein, mercaptosuccinic acid (MSA) protected ultrasmall alloy AuAg nanoclusters were prepared, and the main product was determined [Au₃Ag₅(MSA)₃]⁻ by electrospray ionization mass spectrometry (ESI-MS). The clusters can not only mediate the decomposition of H₂O₂ to generate hydroxyl radicals, but is also able to mediate the reduction of nicotinamide adenine dinucleotide (NAD) to its reduced form of NADH. This is the first time that the atomically precise metal nanoclusters were used to mediate the coenzyme reduction. The preliminary mechanistic insights imply the reaction to be driven by the hydrogen bonding between the carboxylic groups (on the surface of MSA) and the amino N–H bonds (on NAD). In this context, the presence of the carboxylic groups, the sub-nanometer size regime (~ 1 nm), and the synergistic effect of the Au-Ag clusters are pre-requisite to the NAD reduction.