Square antiprismatic molecular geometry

In chemistry, the square antiprismatic molecular geometry describes the shape of compounds where eight atoms, groups of atoms, or ligands are arranged around a central atom, defining the vertices of a square antiprism.[1] This shape has D_4d symmetry and is one of the three common shapes for octacoordinate transition metal complexes, along with the dodecahedron and the bicapped trigonal prism.[2][3]

Square antiprismatic molecular geometry
Examples	XeF^2− 8, ReF^− 8
Point group	D4d
Coordination number	8
μ (Polarity)	0

Like with other high coordination numbers, eight-coordinate compounds are often distorted from idealized geometries, as illustrated by the structure of Na₃TaF₈. In this case, with the small Na⁺ ions, lattice forces are strong. With the diatomic cation NO⁺, the lattice forces are weaker, such as in (NO)₂XeF₈, which crystallizes with a more idealized square antiprismatic geometry.

• 
  The distorted square antiprismatic [TaF₈]³⁻ anion in the Na₃TaF₈ lattice.[4]
• 
  The square antiprismatic [XeF₈]²⁻ anion in the lattice of nitrosonium octafluoroxenate(VI), (NO)₂XeF₈.[5]
• 
  Structure of the B_i8²⁺ cluster in the [Bi₈](GaCl₄)₂.

Examples

• XeF²⁻
  ₈
• IF⁻
  ₈
• ReF⁻
  ₈

References

1. D. L. Kepert (1978). "Aspects of the Stereochemistry of Eight-Coordination". Progress in Inorganic Chemistry. 24: 179–249. doi:10.1002/9780470166253.ch4.
2. Jeremy K. Burdett; Roald Hoffmann; Robert C. Fay (1978). "Eight-Coordination". Inorganic Chemistry. 17 (9): 2553–2568. doi:10.1021/ic50187a041.
3. Wells A.F. (1984) Structural Inorganic Chemistry 5th edition Oxford Science Publications ISBN 0-19-855370-6
4. Langer, V.; Smrčok, L.; Boča, M. (2010). "Redetermination of Na₃TaF₈". Acta Crystallographica Section C. 66: pi85–pi86. doi:10.1107/S0108270110030556.CS1 maint: uses authors parameter (link)
5. Peterson, W.; Holloway, H.; Coyle, A.; Williams, M. (Sep 1971). "Antiprismatic Coordination about Xenon: the Structure of Nitrosonium Octafluoroxenate(VI)". Science. 173 (4003): 1238–1239. Bibcode:1971Sci...173.1238P. doi:10.1126/science.173.4003.1238. ISSN 0036-8075. PMID 17775218.