title: Theoretical and Synthetic Study on the Existence, Structures, and Bonding of the Halide-Bridged [B2X7](-) (X = F, Cl, Br, I) Anions.
abstract: While hydrogen bridging is very common in boron chemistry, halogen bridging is rather rare. The simplest halogen-bridged boron compounds are the [B2X7](-) anions (X = F, Cl, Br, I), of which only [B2F7](-) has been reported to exist experimentally. In this paper a detailed theoretical and synthetic study on the [B2X7](-) anions is presented. The structures of [B2X7](-) anions have been calculated at the MP2/def2-TZVPP level of theory, and their local minima have been shown to be of C2 symmetry in all cases. The bonding situation varies significantly between the different anions. While in [B2F7](-) the bonding is mainly governed by electrostatics, the charge is almost equally distributed over all atoms in [B2I7](-) and additional weak iodine···iodine interactions are observed. This was shown by an atoms in molecules (AIM) analysis. The thermodynamic stability of the [B2X7](-) anions was estimated in all phases (gas, solution, and solid state) based on quantum-chemical calculations and estimations of the lattice enthalpies using a volume-based approach. In the gas phase the formation of [B2X7](-) anions from [BX4](-) and BX3 is favored in accord with the high Lewis acidity of the BX3 molecules. In solution and in the solid state only [B2F7](-) is stable against dissociation. The other three anions are borderline cases, which might be detectable under favorable conditions. However, experimental attempts to identify [B2X7](-) (X = Cl, Br, I) anions in solution by (11)B NMR spectroscopy and to prepare stable [PNP][B2X7] salts failed.
authors.affiliation: Fakultät für Mathematik und Naturwissenschaften, Anorganische Chemie, Bergische Universität Wuppertal , 42119 Wuppertal, Germany.
authors.forename: Philipp
authors.initials: P
authors.lastname: Bertocco
authors.affiliation: Fakultät für Mathematik und Naturwissenschaften, Anorganische Chemie, Bergische Universität Wuppertal , 42119 Wuppertal, Germany.
authors.forename: Christoph
authors.initials: C
authors.lastname: Bolli
authors.affiliation: Fakultät für Mathematik und Naturwissenschaften, Anorganische Chemie, Bergische Universität Wuppertal , 42119 Wuppertal, Germany.
authors.forename: Bruno A
authors.initials: BA
authors.lastname: Correia Bicho
authors.affiliation: Fakultät für Mathematik und Naturwissenschaften, Anorganische Chemie, Bergische Universität Wuppertal , 42119 Wuppertal, Germany.
authors.forename: Carsten
authors.initials: C
authors.lastname: Jenne
authors.affiliation: Fakultät für Mathematik und Naturwissenschaften, Anorganische Chemie, Bergische Universität Wuppertal , 42119 Wuppertal, Germany.
authors.forename: Berrin
authors.initials: B
authors.lastname: Erken
authors.affiliation: Laboratory of Inorganic Chemistry, University of Oulu , P.O. Box 3000, 90014 Oulu, Finland.
authors.forename: Risto S
authors.initials: RS
authors.lastname: Laitinen
authors.affiliation: Fakultät für Mathematik und Naturwissenschaften, Anorganische Chemie, Bergische Universität Wuppertal , 42119 Wuppertal, Germany.
authors.forename: Helene A
authors.initials: HA
authors.lastname: Seeger
authors.affiliation: Fakultät für Mathematik und Naturwissenschaften, Anorganische Chemie, Bergische Universität Wuppertal , 42119 Wuppertal, Germany.|Laboratory of Inorganic Chemistry, University of Oulu , P.O. Box 3000, 90014 Oulu, Finland.
authors.forename: Teemu T
authors.initials: TT
authors.lastname: Takaluoma