CONSPECTUS: Silver and gold molecular nanoparticles (mNPs) are a relatively new class of molecular materials of fundamental interest. They are high-nuclearity metal−organic compounds, with ligated metal cores, where the different character of bonding in the ligand shell and metal core gives rise to many of the unique properties of these materials. Research has primarily focused on gold mNPs, due to their good stability and the ease with which they may be synthesized and processed. To understand these materials as a general class, however, it will be necessary to broaden research efforts to other metals. Gold and silver are isoelectronic and have the same atomic radius, making the comparison of gold and silver mNPs attractive. The optical and chemical differences of the two metals provide useful contrasts, however, as well as a means to access a wider range of properties. In this Account, we focus on the synthesis, structure, and reactivity of silver mNPs. First, we review the origins and history of the field, from the ill-defined gas-phase metal clusters of the 1980s to the precisely defined mNPs of 1996 and onward. Next, we discuss the role of silver as a complement to gold mNPs in the effort to generalize lessons learned from either material and extend them into new metals. The synthesis of silver mNPs is covered in some detail, noting the choices made as the chemistry and the materials were developed. The importance of coordinating solvents and thermodynamic stability are also noted. The need to reduce solvent use is discussed and a new approach to achieving this goal is presented. Next, the structures of silver mNPs are discussed, including the Ag 44 and Ag 17 archetypes, and focusing on the successful de novo structure prediction of the latter. Structure and prediction of ligand shell motifs are also discussed. Finally, the postsynthetic chemistry and reactivity of silver mNPs are presented, including some of the first efforts to elucidate reaction mechanisms, beginning in 2012. Silver nanoparticles are gaining in popularity, particularly compared with gold, as the potential for silver to make a technological and economic impact is recognized. The superior optical properties of silver already make it a valuable material for plasmonics, but this may also translate to molecular species for nonlinear optics, sensors, and optoelectronics. The higher reactivity may also lead to a greater diversity of chemistry for silver compared to gold, including as an important broad-spectrum antimicrobial. Conversely, the "ultrastability" of the Ag 44 archetype has already enabled unprecedented scale up with molecular precision, and may lead to the first industrial-scale production of metal mNPs. Clearly, silver mNPs are one of the most promising and significant new materials being studied today.

M 4 Ag 44 (p-MBA) 30 molecular nanoparticles, where M is an alkali metal, have recently been shown to have exceptional stability, which confers unique traits to this molecule. In particular, the synthesis is straightforward, produces a truly single-sized molecular product, and has a quantitative yield. Here we describe in detail the results of experimental and theoretical studies on the synthesis, structure, stability, and electronic and optical properties of M 4 Ag 44 (p-MBA) 30 , including ESI-MS, NMR, optical absorption, IR, TGA, and other measurements as well as DFT and TDDFT calculations. This work deepens our understanding of this important Ag molecule, which should facilitate its use in a wide range of fundamental studies and applications.

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Metal heteroatom substitution in molecular nanoparticles offers a unique opportunity to study alloying with an unprecedented level of detail and control, which may be important for applications in optics, electronics, medicine, and catalysis. Many examples of single- or few-heteroatom substitutions exist in molecular nanoparticles, but true alloys with compositions varying over a wide range of substitutions are rare. Here, we study M4Au x Ag44–x (p-MBA)30 alloy nanoparticles as a model system, where M is a countercation and p-MBA is a p-mercaptobenzoic acid ligand, and where 0 ≤ x ≤ 12 represents the full range of possible compositions, all of which are stable in solution. Synthetic reactions produced M4Au x Ag44–x (p-MBA)30 alloy nanoparticle products whose compositions were found to be a complex function of the reaction mixture composition. Postsynthetic reactions showed that oxidation of M4Au x Ag44–x (p-MBA)30 nanoparticles was slowed in a monotonic fashion by the addition of gold atoms. Density functional theory provided insights into the variation in the chemistry, electronic structure, and reactivity of M4Au x Ag(44–x)(p-MBA)30 nanoparticle alloys as a function of composition and showed that the stabilization mechanism was the result of the electrophilicity of the gold atoms, which polarized the metal core. An oxidation reaction mechanism was proposed based on these experimental and computational results, which involved the octahedrally located silver atoms.

The synthesis and structure of mixed gold/silver M 4Au12Ag32(p-MBA)30 bimetallic monolayer-protected clusters is reported and compared to that of silver M 4Ag44(p-MBA)30 monolayer-protected clusters (M = Na, Cs).