Synthesis, Crystal Structure, and Optical Properties of the Barium meta-Pertechnetate Ba[TcO4]2

High-quality single crystals of the colorless title compound Ba[99TcO4]2 were prepared from radioactive, isotope-pure 99TcO2 by oxidation with H2O2 in an aqueous ammonia solution and subsequent cation metathesis using an ion-exchange column. Single-crystal X-ray diffraction revealed Ba[TcO4]2 to crystallize isotopically to Ba[MnO4]2 in the orthorhombic space group Fddd with the lattice parameters a = 747.34(5) pm, b = 1244.08(7) pm, and c = 1511.23(9) pm for Z = 8. Its crystal structure features [BaO12]22– icosahedra surrounded by ten [TcO4]− tetrahedra connected via two edges and eight corners. The optical properties of Ba[TcO4]2 investigated by UV-vis spectroscopy show the broad O2– → Tc7+ ligand-to-metal charge transfer (2p → 4d) process within these discrete [TcO4]− anions at an absorption maximum of around 350 nm.