Ag5GeO4, das erste subvalente tern�re Silberoxid

Where are the Excess Electrons in Subvalent Compounds? The Case of Ag7Pt2O7

Subvalent compounds raise the question of where those valence electrons not belonging to chemical bonds are. In the limiting case of Ag7Pt2O7, there is just one-electron excess in the chemical formula requiring the presence of Ag atoms with oxidation states below +1, assuming conventional Pt4+ and O2- ions. Such a situation challenges the understanding of the semiconducting and diamagnetic behavior observed in this oxide. Previous explanations that localize pairwise the electron excess in tetrahedral Ag4 interstices do not suffice in this case, since there are six silver tetrahedral voids and only an excess of nine electrons in the unit cell. Here, we provide an alternative explanation for the subvalent nature of this compound by combining interatomic distances, electron density-based descriptors, and orbital energetic analysis criteria. As a result, Ag atoms that do not participate in their valence electron are revealed. We identify excess electrons located in isolated subvalent silver clusters with electron-deficient multicenter bonds resembling pieces of metallic bonding in fcc-Ag and Ag7Pt2 alloy. Our analysis of the electronic band structure also supports the multicenter bonding picture. This combined approach from the real and reciprocal spaces reconciles existing discrepancies and is key to understanding the new chemistry of silver subvalent compounds.

Ag6Cl4: the first silver chloride with rare Ag6 clusters from an ab initio study

The first diamagnetic semiconducting silver subhalide, Ag₆Cl₄, featuring rare subvalent Ag₆ clusters with 2e^--6c bonds, 1D argentophilic d¹⁰-d¹⁰ intercluster interactions and 3D ionic connectivity ensured by Cl atoms has been predicted employing Density Functional Theory. Ag₆Cl₄ carries all unique features of Ag⁺ so far observed only in selected metal-rich oxides and as such represents an important addition to the discussion of the special bonding properties of silver with a filled d¹⁰-shell. Having appreciable formation enthalpy and dynamical stability, Ag₆Cl₄ should be in principle possible to synthesize as a metastable phase relative to AgCl.

Reply to the ‘Comment on “Uncommon structural and bonding properties in Ag16B4O10” by A. Lobato, Miguel Á. Salvadó, and J. Manuel Recio, Chem. Sci., 2021, 12, DOI: 10.1039/D1SC02152D

Where are the excess electrons in Ag₁₆B₄O₁₀?

Idiosyncratic Ag7Pt2O7: An Electron Imprecise yet Diamagnetic Small Band Gap Oxide

The seminal qualitative concepts of chemical bonding, as presented by Walter Kossel and Gilbert Newton Lewis back in 1916, have lasting general validity. These basic rules of chemical valence still serve as a touchstone for validating the plausibility of composition and constitution of a given chemical compound. We report on Ag7Pt2O7, with a composition that violates the basic rules of chemical valence and an exotic crystal structure. The first coordination sphere of platinum is characteristic of tetravalent platinum. Thus, the electron count corresponds to Ag7Pt2O7*e-, where excess electrons are associated with the silver substructure. Such conditions given, it is commonly assumed that the excess electrons are either itinerant or localized in Ag-Ag bonds. However, the material does not show metallic conductivity, nor does the structure feature Ag-Ag pairs. Instead, the excess electrons organize themselves in 2e-4c bonds within the silver substructure. This subvalent silver oxide reveals a new general facet pertinent to silver chemistry.

Uncommon structural and bonding properties in Ag16B4O10

Ag₁₆B₄O₁₀ has been obtained as a coarse crystalline material via hydrothermal synthesis, and was characterized by X-ray single crystal and powder diffraction, conductivity and magnetic susceptibility measurements, as well as by DFT based theoretical analyses. Neither composition nor crystal structure nor valence electron counts can be fully rationalized by applying known bonding schemes. While the rare cage anion (B₄O₁₀)⁸⁻ is electron precise, and reflects standard bonding properties, the silver ion substructure necessarily has to accommodate eight excess electrons per formula unit, (Ag⁺)₁₆(B³⁺)₄(O²⁻)₁₀ × 8e⁻, rendering the compound sub-valent with respect to silver. However, the phenomena commonly associated with sub-valence metal (partial) structures are not perceptible in this case. Experimentally, the compound has been found to be semiconducting and diamagnetic, ruling out the presence of itinerant electrons; hence the excess electrons have to localize pairwise. However, no pairwise contractions of silver atoms are realized in the structure, thus excluding formation of 2e–2c bonds. Rather, cluster-like aggregates of an approximately tetrahedral shape exist where the Ag–Ag separations are significantly smaller than in elemental silver. The number of these subunits per formula is four, thus matching the required number of sites for pairwise nesting of eight excess electrons. This scenario has been corroborated by computational analyses of the densities of states and electron localization function (ELF), which clearly indicate the presence of an attractor within the shrunken tetrahedral voids in the silver substructure. However, one bonding electron pair of s and p type skeleton electrons per cluster unit is extremely low, and the significant propensity to form and the thermal stability of the title compound suggest d¹⁰–d¹⁰ bonding interactions to strengthen the inter-cluster bonding in a synergistic fashion. With the present state of knowledge, such a particular bonding pattern appears to be a singular feature of the oxide chemistry of silver; however, as indicated by analogous findings in related silver oxides, it is evolving as a general one.

Ag₁₆B₄O₁₀, obtained via hydrothermal synthesis, displays an unprecedented bonding scheme, hosting excess electrons localized pairwise in cluster-like silver subunits.