Fluxional seven-coordinated fluorido- and oxofluoridotantalates.

Seven-coordinated (NH4)2TaF7, Rb2TaF7 and Rb3TaOF6 were synthesized in single-crystal form and their structures were determined. A monocapped trigonal prism (CTP) or a pentagonal bipyramid (PB) of the TaF72- anion are stereochemically nonrigid and coexist in the first two compounds as a result of strong intraspheric dynamics. Upon cooling, tetragonal Rb2TaF7 undergoes a first-order phase transition at 145 K and the seven-coordinated polyhedron transforms into a regular CTP. The seven-coordinated polyhedron in (NH4)2TaF7 approaches the PB configuration as the temperature decreases. Cubic elpasolite-like Rb3TaOF6 is characterized by the simultaneous two-state coexistence of TaOF63- of the PB shape as rigidly reoriented and as fluxional. In the former case, the central atom is disordered over the octahedron in the unit cell, allowing the determination of the short Ta-O distance, whereas in the latter case, tantalum remains in the polyhedron center, resulting in synchronous Ta-O and Ta-F stretching vibrations appearing as the infrared band at 723 cm-1.

Electronic Structure and Energy Transfer in Europium(III)-Ciprofloxacin Complexes: A Theoretical Study.

The structure and ligand-localized excited states of [Eu(cfqH) (cfq)(H2O)4]Cl2 (cfqH is ciprofloxacin) are studied by XMCQDPT2/CASSCF with full geometry optimization. The complex includes one anionic and one zwitterionic ligand. Two low-lying triplet states, both localized on the anionic ligand, are found. One of them has sufficient energy to transfer to the (5)D1 sublevel of Eu(3+), because its T-S0 vertical transition energy is equal (or very close) to the (7)F0-(5)D1 Eu(3+) excitation energy. The other triplet state has a very small S0-T1 gap, which favors fast nonradiative relaxation. Two other triplet states are localized on the zwitterionic ligand. One low-lying excited singlet state (S1) is localized on the anionic ligand; the other excited singlet is localized on the zwitterionic one. Spin-orbit coupling constants were calculated for the relaxed geometry of each state (ground state, two low-lying triplets, and one low-lying excited singlet) by spin-orbit configuration interaction (CI) with Pauli-Breit Hamiltonian. Large spin-orbit coupling constants between S1 and both triplets together with small energy gaps are indicative of fast intersystem crossing (ISC) from the excited singlet state to the triplet manifold. This ISC process is followed by energy transfer from the ligand-localized triplet states to the (5)D1 sublevel of Eu(3+). However, relatively large spin-orbit coupling constants between S0 and one of the triplet states together with the small T-S0 energy gap shows that this state can decay without transferring its energy to Eu(3+). This mechanism is expected to be common for other Ln(3+)-fluoroquinolone complexes.