The novel seven-coordinated BiF72- and SbF72- 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 degrees C. Attempts failed to prepare the corresponding BiF83- salts or Li2BiF7 under similar conditions. The [N(CH3)(4)](2)BiF7 Salt was obtained by the combination of excess N(CH3)(4)F with BiF5 in CH3CN solution at -31 degrees C. The (NO2)(2)BiF7 salt was prepared from BiF5 and a large excess of liquid FNO at -78 degrees C and decomposes at room temperature to NOBiF6 and FNO. The corresponding Cs2SbF7, K2SbF7, and [N(CH3)(4)](2)SbF7 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 BiF72-and SbF72- 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-5h symmetry, similar to those of IF7 and TeF7-, which are isoelectronic with SbF72-. Although our theoretical calculations indicate that AsF72- is also vibrationally stable, experiments to prepare this dianion were unsuccessful.