7-COORDINATED PNICOGENS - SYNTHESIS AND CHARACTERIZATION OF THE SBF72- AND BIF72- DIANIONS AND A THEORETICAL-STUDY OF THE ASF(7)(2-)DIANION

The novel seven-coordinated BiF72- and SbF72- dianions have been prepa red and characterized. The Cs2BiF7, Rb2BiF7, K2BiF7 and Na2BiF7 salts were obtained in high yield by heating BiF5 with an excess of the corr esponding alkali metal fluorides to about 250 degrees C. Attempts fail ed to prepare the corresponding BiF83- salts or Li2BiF7 under similar conditions. The [N(CH3)(4)](2)BiF7 Salt was obtained by the combinatio n of excess N(CH3)(4)F with BiF5 in CH3CN solution at -31 degrees C. T he (NO2)(2)BiF7 salt was prepared from BiF5 and a large excess of liqu id FNO at -78 degrees C and decomposes at room temperature to NOBiF6 a nd FNO. The corresponding Cs2SbF7, K2SbF7, and [N(CH3)(4)](2)SbF7 salt s were also synthesized in a similar fashion, but Na2SbF7 was not form ed. The pronounced fluoride ion affinity of SbF6- was further demonstr ated by the formation of some Cs2SbF7 when dry CsF and CsSbF6 were bal l-milled at room temperature. The BiF72- and SbF72- anions, which are the first examples of binary pnicogen compounds with coordination numb ers in excess of six, were characterized by vibrational spectroscopy a nd ab initio electronic structure calculations. They possess pentagona l bipyramidal, highly fluxional structures of D-5h symmetry, similar t o those of IF7 and TeF7-, which are isoelectronic with SbF72-. Althoug h our theoretical calculations indicate that AsF72- is also vibrationa lly stable, experiments to prepare this dianion were unsuccessful.