A 70-Year-Old Mystery in Technetium Chemistry Explained by the New Technetium Polyoxometalate [H₇ O₃ ]₄ [Tc₂₀ O₆₈...

[H₇ O₃ ]₄ [Tc₂₀ O₆₈ ] ⋅ 4H₂ O [1] was prepared from an aqueous Tc₂ O₇ solution concentrated over anhydrous H₂ SO₄ . [Tc₂₀ O₆₈ ]^4- is the first polyanionic species to be reported for Tc. The unit cell contains one centrosymmetric [Tc₂₀ O₆₈ ]^4- polyanion as well as hydronium ions and water molecules. The core of the structure consists of four Tc(V)O₆ octahedra that form a square Tc₄ O₄ ring. The four Tc(V)O₆ octahedra are decorated by sixteen Tc(VII)O₄ tetrahedra. Calculations show the bonding within the Tc₄ O₄ 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.