A 70-year-old mystery in technetium chemistry explained by the new technetium polyoxometalate [H${}_7$O${}_3$]${}_4$[Tc${}_{20}$O${}_{68}$] · 4H${}_2$O

[H${}_7$O${}_3$]${}_4$[Tc${}_{20}$O${}_{68}$] · 4H${}_2$O [1] was prepared from an aqueous Tc${}_2$O${}_7$ solution concentrated over anhydrous H${}_2$SO${}_4$. [Tc${}_{20}$O${}_{68}$]${}^{4-}$ is the first polyanionic species to be reported for Tc. The unit cell contains one centrosymmetric [Tc${}_{20}$O${}_{68}$]${}^{4-}$ polyanion as well as hydronium ions and water molecules. The core of the structure consists of four Tc(V)O${}_6$ octahedra that form a square Tc${}_4$O${}_4$ ring. The four Tc(V)O${}_6$ octahedra are decorated by sixteen Tc(VII)O${}_4$ tetrahedra. Calculations show the bonding within the Tc${}_4$O${}_4$ ring to consist of a 3-center bond formed between each neighboring pair of Tc atoms and their bridging oxygen. Calculations also indicate that a strong d→d electronic transition at 513 nm is the origin of the red color of [1]. The characterization of red HTcO${}_4$ solutions by X-ray absorption spectroscopy has complemented the description of this compound in aqueous solution. The formation mechanisms in solution, including the possible role of technetium's radioactivity in the formation of [1], are discussed. (© 2021 Wiley‐VCH GmbH)