In a very interesting article by Giese and Seppelt from 1994  the question of the preferred coordination geometry for the coordination number 7 is explored. Until then, it has been shown that the pentagonal bipyramid is preferred for main group elements (for instance in the compound IF7), although this geometry results in an overall slightly higher ligand repulsion than the alternative arrangements according to a capped octahedron or capped tigonal prism, respectively.
In this article they raised the question if this is also valid for transition elements, and the ideal candidate to answer this question would be the homoleptic compound ReF7. Unfortunately, for various reasons it is very difficult to determine the crystal structure of ReF7.
Alternatively, they synthesized several ionic compounds comprising of anions with transition elements with the coordination number 7 and varied the nature of the cation to exclude lattice energy effects that might influence the arrangement of the ligands.
The results were as follows:
The anions in Cs+MoF7– (see Fig. 1), Cs+WF7–, NO2+MoF7– ∙ CH3CN, and C11H24N+MoF7– (C11H24N+ = 1,1,3,3,5,5-hexamethylpiperidinium) (all belong to the cubic crystal system) as well as (H3C)4N+MoF7– (tetragonal, space group P4/nmm, ) form a capped octahedron coordination polyhedron. The latter result is of special interest as the 7 fluorine atoms around Tellurium in (H3C)4N+TeF7– (again space group P4/nmm) form a pentagonal bipyramid instead.
Fig. 1: The crystal structure of Cs+MoF7– (space group Pa-3, ICSD deposition number = 78390); Cs = purple, Mo = gray, F = green. (Image made with VESTA ).
According to the authors, it can therefore be concluded that neither the lattice type nor the crystal packing have an influence on the different structures of the anions.
Now, what will happen, if one of the 7 fluorine ligands is exchanged with a larger atom? As expected, a pentagonal bipyramid is then realised, as in Cs+ReOF6– (space group P21/a), where the larger ligand occupies one of the two axial positions (see Fig. 2).
Fig. 2: The crystal structure of Cs+ReOF6– (space group P21/a, ICSD deposition number = 78392); Cs = purple, Re = gray, O = red, F = green. (Image made with VESTA ).
I have to admit that I was too lazy to check if the structure of ReF7 is known by now. In fact – thanks to Robert McMeeking (from The Chemical Database Service/CrystalWorks, @cds_daresbury) for the hint – the crystal structure of ReF7 was published in Science only a single(!) day after the publication of the Angewandte paper by Giese & Seppelt: ReF7 builds a (slightly distorted) pentagonal bipyramid , see Fig. 3!
Fig. 3: The crystal structure of ReF7 (space group C-1, ICSD deposition number = 78311); Re = gray, F = green. (Image made with VESTA ).
 S. Giese, K. Seppelt, Angew. Chem. Int. Ed Engl. 1994, 33, 461. http://dx.doi.org/10.1002/anie.199404611
 K. Momma and F. Izumi, “VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data,” J. Appl. Crystallogr. 2011, 44, 1272-1276).
 T. Vogt, A. N. Fitch, J. K. Cockcroft, Science 1994, 263, 1265.