Square antiprismatic molecular geometry

Short description: Shape of molecular compounds

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 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.

* 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.

## Contents

* 1 Examples
* 2 Square prismatic geometry and cubic geometry
* 3 References
* 4 External links

## Examples

* XeF2−8
* IF−8
* ReF−8

## 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]

## References

1. ↑ Kepert, D.L. (1978). Aspects of the Stereochemistry of Eight-Coordination. Progress in Inorganic Chemistry. 24. pp. 179–249. doi:10.1002/9780470166253.ch4. ISBN 9780470166253.
2. ↑ 2.0 2.1 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. doi:10.1107/S0108270110030556. PMID 20814090. Bibcode: 2010AcCrC..66I..85L.
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. doi:10.1126/science.173.4003.1238. ISSN 0036-8075. PMID 17775218. Bibcode: 1971Sci...173.1238P.
6. ↑ Brown, D.; Easey, J. F.; Rickard, C. E. F. (1969). "Cubic co-ordination: crystal structure of sodium octafluoroprotactinate(V)" (in en). Journal of the Chemical Society A: Inorganic, Physical, Theoretical: 1161. doi:10.1039/j19690001161. ISSN 0022-4944. http://xlink.rsc.org/?DOI=j19690001161.

## External links

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

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Molecular geometry (VSEPR)

Coordination number 2|

* Linear
* Bent

Coordination number 3|

* Trigonal planar
* Trigonal pyramidal
* T-shaped

Coordination number 4|

* Tetrahedral
* Square planar
* Seesaw

Coordination number 5|

* Trigonal bipyramidal
* Square pyramidal
* Pentagonal planar

Coordination number 6|

* Octahedral
* Trigonal prismatic
* Pentagonal pyramidal

Coordination number 7|

* Pentagonal bipyramidal
* Capped octahedral
* Capped trigonal prismatic

Coordination number 8|

* Square antiprismatic
* Dodecahedral
* Bicapped trigonal prismatic

Coordination number 9|

* Tricapped trigonal prismatic
* Capped square antiprismatic

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