Atomically precise gold/silver clusters protected by organic ligands L, [(Au/Ag)xLy]z, have gained increasing interest as building units of functional materials because of their novel photophysical and physicochemical properties. The properties of [(Au/Ag)xLy]z are intimately associated with the quantized electronic structures of the metallic cores, which can be viewed as superatoms from the analogy of naked Au/Ag clusters. Thus, establishment of the correlation between the geometric and electronic structures of the superatomic cores is crucial for rational design and improvement of the properties of [(Au/Ag)xLy]z. This review article aims to provide a qualitative understanding on how the electronic structures of [(Au/Ag)xLy]z are affected by geometric structures of the superatomic cores with a focus on three factors: size, shape, and composition, on the basis of single‐crystal X‐ray diffraction data. The knowledge accumulated here will constitute a basis for the development of ligand‐protected Au/Ag clusters as new artificial elements on a nanometer scale.

A representative thiolate (RS)-protected gold cluster, Au25(SR)18, shows a fingerprint-like characteristic spectral profile regardless of the R-groups, reflecting the common motif of the structural backbone made of Au and S: an icosahedral Au13 core fully protected by six staple units of Au2(SR)3. On the other hand, we reported in 2006 that an Au25(SPG)18 cluster (PGSH = N-(2-mercaptopropionyl)­glycine) exhibited an optical absorption spectrum significantly different from that of the conventional Au25(SR)18, suggesting the formation of a nonicosahedral Au core. Here, we investigated the structure of Au25(SPG)18 by UV–vis spectroscopy, extended X-ray absorption fine structure analysis and density functional theory calculations. Spectroscopic results indicated that Au25(SPG)18 has a face-centered cubic (FCC) Au core. We proposed a model structure formulated as Au15(SPG)4[Au2(SPG)3]2[Au3(SPG)4]2 in which an Au15(SPG)4 core with an FCC motif is protected by two types of staples with different lengths, Au2(SPG)3 and Au3(SPG)4. The formation of an FCC-based Au core is attributed to bulkiness around the α-carbon of the PGS ligand.

We present reactions of size-selected free silicate, Mg l SiO m − , and silica, Si n O m − , cluster anions with a H 2 O molecule focusing on H 2 O adsorption. It was found that H 2 O adsorption to Mg l SiO m− with l = 2 and 3 (m = 4−6) is always followed by molecular oxygen release, whereas reactivity of the clusters with l = 1 (m = 3−5) was found to be much lower. On the contrary, in the reaction of SiH 2 O adduct is observed as a major reaction product. Larger and oxygenrich clusters tend to exhibit higher reactivity; the rate constants of the adsorption reaction are 2 orders of magnitude larger than those of CO adsorption previously reported. DFT calculations revealed that H 2 O is dissociatively adsorbed on Si n O m − to form two SiO 3 (OH) tetrahedra. The site selectivity of H 2 O adsorption is governed by the location of the singly occupied molecular orbital (SOMO) on Si n O m − . The present findings give molecular-level insights into H 2 O adsorption on silica and silicate species in the interstellar environment.

Cyclohexanethiolate-protected gold cluster Au 25 (c-HT) 18 was isolated for the first time as a minor byproduct of [Au 23 (c-HT) 16 ] ¹. Single-crystal X-ray structure analysis revealed that Au 25 (c-HT) 18 has an icosahedral Au 13 core, in sharp contrast to the face-centered cubic cores in major products such as [Au 23 (c-HT) 16 ] ¹ and Au 28 (c-HT) 20. Au 25 (c-HT) 18 prefers a neutral state to an anionic state, probably because the Au 13 core is forced to deform due to the steric hindrance between the c-HT ligands.