Square antiprismatic molecular geometry
, ReF
Point groupD4d
Coordination number8
μ (Polarity)0

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 D4d 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]

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.


Square prismatic geometry and cubic geometry

Square prismatic geometry (D4h) is much less common compared to the square antiprism. An example of a molecular species with square prismatic geometry (a slightly flattened cube) is octafluoroprotactinate(V), [PaF8]3–, as found in its sodium salt, Na3PaF8.[6] While local cubic 8-coordination is common in ionic lattices (e.g., Ca2+ in CaF2), and some 8-coordinate actinide complexes are approximately cubic, there are no reported examples of rigorously cubic 8-coordinate molecular species. A number of other rare geometries for 8-coordination are also known.[2]


  1. ^ D. L. Kepert (1978). "Aspects of the Stereochemistry of Eight-Coordination". Progress in Inorganic Chemistry. 24: 179–249. doi:10.1002/9780470166253.ch4. ISBN 9780470166253.
  2. ^ a b 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 (9): pi85–pi86. doi:10.1107/S0108270110030556. PMID 20814090.
  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. S2CID 22384146.
  6. ^ Brown, D.; Easey, J. F.; Rickard, C. E. F. (1969). "Cubic co-ordination: crystal structure of sodium octafluoroprotactinate(V)". Journal of the Chemical Society A: Inorganic, Physical, Theoretical: 1161. doi:10.1039/j19690001161. ISSN 0022-4944.