A 70-Year-Old Mystery In Technetium Chemistry Explained By The New Technetium Polyoxometalate [H7o3](4)[Tc20o68] . 4h(2)O

[H7O3](4)[Tc20O68] . 4H(2)O [1] was prepared from an aqueous Tc2O7 solution concentrated over anhydrous H2SO4. [Tc20O68](4-) is the first polyanionic species to be reported for Tc. The unit cell contains one centrosymmetric [Tc20O68](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 Tc4O4 ring. The four Tc(V)O-6 octahedra are decorated by sixteen Tc(VII)O-4 tetrahedra. Calculations show the bonding within the Tc4O4 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 HTcO4 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.