Octahedral/pentagonal bipyramidal

The 6-, 7-, and 8-coordinate complexes are almost invariably octahedral, pentagonal bipyramidal, and dodecahedral, respectively. 

Hexavalent. The majority of An(VI) coordination chemistry with N-donors has been explored with the uranyl cation, 50i. Stable adducts with the hgands discussed in the tri- and tetravalent complexes have been described, for example, U02X2L (X = halide, OR, NO3, RCO2). The coordination numbers observed for these complexes are typically 6, 7, or 8 with octahedral, pentagonal bipyramidal, or hexagonal bipyramidal geometries, respectively. Neutral and anionic thiocyanates have also been isolated, for example [U02(NCS)j2- yH20(x = 2 5). 

Whereas the coordination chemistry of thorium is concerned with only the +4 oxidation state, that of uranium covers oxidation states -1-3 to -1-6. For U(VI), the linear [U02] unit is generally present within the complex and fra -octahedral, pentagonal bipyramidal and hexagonal bipyramidal complexes are usual. For other oxidation states, the coordination polyhedron is essentially determined by the spatial requirements of the ligands rather than electronic factors, and the large size of the U centre allows high coordination numbers to be attained. Complexes involving different oxidation states and coordination numbers include ... 

Fig. 9 Face-dual relationship of bonding and nonbonding densities in different coordination complexes. The nonbonding densities of octahedral, pentagonal bipyramidal (simplified as bicone), and square-antiprismatic (simplified as cylinder) geometries can be approximated as cube, cylinder, and bicone, respectively, illustrating a face-dual relationship. The face-dual plots are made with visvis [27] visualization library...

Square planar Trigonal bipyramidal Square pyramidal Octahedral Pentagonal bipyramidal Square antiprismatic Tetrahedral... 

XeFs 49.5 Distorted octahedral (fluxional) Pentagonal bipyramidal or capped oct edral (1)... 

Key Colour % indicates preparation but no report of colour) mp/°C (na indicates value not reported) coordination 9 ttp = tricapped trigonal prismatic 8 d = dodecahedral 8 sa = square antiprismatic 8 btp = bicapped trigonal prismatic 8,7 = mixed 8- and 7-coordination (SrBr2 structure) 7 cc = capped octahedral 7 pbp = pentagonal bipyramidal 6 o = octahedral 6 och = octahedral chain, 6 ol = octahedral layered. 

Figure 31.7 (a) The octahedral anion in Cs2[U02CU]. (b) Pentagonal bipyramidal coordination of U in dinuclear [U02(02CMe)2L]2 (L = OPPhs, OAsPhs). (c) Hexagonal bipyramidal coordination of U in uranyl nitrate, U02(N03)2-6H20. 

It is noteworthy that the to-bonded structure for ArF6 differs from that predicted by VSEPR theory. ArF6 is predicted to be of octahedral (Oh) symmetry, with three mutually perpendicular F i- Ar -h F triads and an s-type lone pair. In contrast, VSEPR predicts a pentagonal bipyramid (or other seven-vertex polyhedron) with some or all F-Ar-F angles less than 90°. The calculated equilibrium structure is in agreement with the co-bonding model. 

ZrM02O7fOH)2(H2O)2] has been prepared by refluxing Zr(Mo04)2 gels in HCl (1—4 mol" and the crystals have been shown to contain cis-octahedral [Mo04(OH)H20] and pentagonal-bipyramidal [Zr 03(0H)2 (02) J units. 

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