A re-interpretation of the structure of the silver borate, Ag16B4O10, in the light of the extended Zintl-Klemm concept.

Abstract

The borate Ag16B4O10 was synthesized at high temperature and at elevated oxygen pressures [Kovalevskiy et al. (2020). Chem. Sci. 11, 962-969]. Its structure consists of [B4O10]8- polyanions (isostructural to P4O10) embedded in an Ag matrix. According to the standard valences Ag+, B3+ and O2-, the formula has an excess of eight e- which the above authors proposed were located, pairwise, in four Ag4 tetrahedra within the silver substructure. That conclusion was based on the semiconducting and diamagnetic properties, as well as the very small `attractors' of the Electron Localization Function (ELF) found at the centre of these Ag4 tetrahedra. However, a large overlap of the projected density of states (DOS) of silver and oxygen indicated possible dispersion interactions between both atomic species. In this article, an alternative description is proposed based on the extended Zintl-Klemm concept. The anion [B4O10]8- can be formulated as Ψ-[N4O10] P4O10, if it is assumed that the eight e- are transferred to the four B atoms, so converting them into Ψ-N, this then makes sense of its similarity with P4O10, [N4(CH2)6], adamantane and tetraisopropyladamantane. When the Ag atoms connect to the O atoms, they arrange as the H atoms do in hexamethylenetetramine (HMTA). If the two lone pairs of each of the bridging O atoms in Ψ-[N4O10] are equated to the C-H bonds in HMTA, then, this same equivalence exists between the C-H bonds and the O-Ag bonds in the compound Ag16B4O10. The 24 Ag atoms surrounding each [B4O10]8- group prolong the sphalerite structure of the borate anion by means of Ag-O bonds which also fit the sphalerite structure formed of AgO. The eight excess electrons might then be distributed between the Ag and the O atoms, so making sense of the mixing of the Ag and O states. The Ag atoms bonded to the O atoms of the [B4O10]8- groups form a coat that interconnects the borate anions through Ag-O bonds. To establish the validity of this new proposal, the study needs to be extended to the compound Ag3B5O9.