Square antiprismatic molecular geometry

Square antiprismatic molecular geometry
Examples	XeF2− 8, ReF− 8
Point group	D4d
Coordination number	8
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 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]}

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 Bi²⁺
  ₈ cluster in the [Bi₈](GaCl₄)₂.

Examples

• XeF²⁻
  ₈
• IF⁻
  ₈
• ReF⁻
  ₈

Square prismatic geometry and cubic geometry

Square prismatic geometry (D_4h) 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]

References

1. Kepert, D.L. (1978). Aspects of the Stereochemistry of Eight-Coordination. Progress in Inorganic Chemistry. Vol. 24. pp. 179–249. doi:10.1002/9780470166253.ch4. ISBN   9780470166253.
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 ed.). 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. Bibcode:2010AcCrC..66I..85L. 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.

External links

• Landskrone Kai. LibreTexts - Chemistry. Section 2.2: Point Groups