Seven-Coordinated Pnicogens. Synthesis and Characterization of the SbF₇^2-and BiF₇^2-Dianions and a Theoretical Study of the AsF₇^2-Dianion

Abstract

The novel seven-coordinated BiF7 2- and SbF7 2- dianions have been prepared and characterized. The Cs2BiF7, Rb2BiF7, K2BiF7, and Na2BiF7 salts were obtained in high yield by heating BiF5 with an excess of the corresponding alkali metal fluorides to about 250 °C. Attempts failed to prepare the corresponding BiF8 3- salts or Li2BiF7 under similar conditions. The [N(CH3)4]2BiF7 salt was obtained by the combination of excess N(CH3)4F with BiF5 in CH3CN solution at −31 °C. The (NO2)2BiF7 salt was prepared from BiF5 and a large excess of liquid FNO at −78 °C and decomposes at room temperature to NOBiF6 and FNO. The corresponding Cs2SbF7, K2SbF7, and [N(CH3)4]2SbF7 salts were also synthesized in a similar fashion, but Na2SbF7 was not formed. The pronounced fluoride ion affinity of SbF6 - was further demonstrated by the formation of some Cs2SbF7 when dry CsF and CsSbF6 were ball-milled at room temperature. The BiF7 2- and SbF7 2- anions, which are the first examples of binary pnicogen compounds with coordination numbers in excess of six, were characterized by vibrational spectroscopy and ab initio electronic structure calculations. They possess pentagonal bipyramidal, highly fluxional structures of D 5 h symmetry, similar to those of IF7 and TeF7 -, which are isoelectronic with SbF7 2-. Although our theoretical calculations indicate that AsF7 2- is also vibrationally stable, experiments to prepare this dianion were unsuccessful.

Fluoride ion transfer and stabilisation of reactive ions

“…Species with higher acceptor properties than the solvent will accept the fluoride. Very unusual inorganic ions have been generated by the method mainly in the laboratories of Christe, Seppelt and Schrobilgen (Scheme 6 [6,[46][47][48][49][50][51][52][53][54][55] …”

Mean amplitudes of vibration of molecules and ions with interhalogen bonds and related species

“…Finally, it is also worth mentioning that the SbF 7 2À and BiF 7 2À anions represent two additional examples of species structurally related to IF 7 [59]. Their mean amplitudes of vibration were also calculated and showed the expected behavior in comparison with the other species discussed in this section [60].…”

Hexafluoro-, heptafluoro-, and octafluoro-salts, and [MnF5n+1]− (n=2, 3, 4) polyfluorometallates of singly charged metal cations, Li+–Cs+, Cu+, Ag+, In+ and Tl+

“…Rb 2 WF 7 was characterized by Raman spectroscopy [53], and [SbF 7 ] 2À and [BiF 7 ] 2À salts by vibrational spectroscopy and ab initio electronic structure calculations [54]. They possess pentagonal bipyramidal structures of D 5h symmetry (Table 10).…”