Square antiprismatic molecular geometry
{{Infobox molecular geometry
| Examples=Nitrosonium octafluoroxenate(VI), {{chem|ReF|8|−}}
| Image_File=Square-antiprismatic-3D-balls.png
| Symmetry_group=D4d
| Atom_direction=8
| Bond_angle=
| mu=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.{{cite book|title=Aspects of the Stereochemistry of Eight-Coordination|pages=179–249|last=Kepert|first=D.L.|series=Progress in Inorganic Chemistry|volume=24|doi=10.1002/9780470166253.ch4|year=1978|isbn=9780470166253}} 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.{{cite journal
| title = Eight-Coordination
| author1 = Jeremy K. Burdett
| author2 = Roald Hoffmann
| author3 = Robert C. Fay
| journal = Inorganic Chemistry
| year = 1978
| volume = 17
| issue = 9
| pages = 2553–2568
| doi = 10.1021/ic50187a041
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.
File:TaF83-Core.png|The distorted square antiprismatic [TaF8]3− anion in the Na3TaF8 lattice.{{cite journal|title=Redetermination of Na3TaF8|author1=Langer, V. |author2=Smrčok, L. |author3=Boča, M. |journal=Acta Crystallographica Section C|year=2010|volume=66|issue=9|pages=pi85–pi86|doi=10.1107/S0108270110030556|pmid=20814090|bibcode=2010AcCrC..66I..85L }}
File:Octafluoroxenate(VI)-3D-balls-A.png|The square antiprismatic [XeF8]2− anion in the lattice of nitrosonium octafluoroxenate(VI), (NO)2XeF8.{{Cite journal|first3 = A.|first2 = H.|first4 = M.|title = Antiprismatic Coordination about Xenon: the Structure of Nitrosonium Octafluoroxenate(VI)|first1 = W.|last2 = Holloway|last3 = Coyle|volume = 173|journal = Science|issue = 4003|pages = 1238–1239|issn = 0036-8075|doi = 10.1126/science.173.4003.1238|last4 = Williams|pmid = 17775218|last1 = Peterson|date = Sep 1971|bibcode = 1971Sci...173.1238P |s2cid = 22384146}}
File:EntryWithCollCode414090.png|Structure of the {{chem|Bi|8|2+}} cluster in the [Bi8](GaCl4)2.
Examples
- Nitrosonium octafluoroxenate(VI)
- {{chem|IF|8|−}}
- {{chem|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.{{Cite journal |last1=Brown |first1=D. |last2=Easey |first2=J. F. |last3=Rickard |first3=C. E. F. |date=1969 |title=Cubic co-ordination: crystal structure of sodium octafluoroprotactinate(V) |url=http://xlink.rsc.org/?DOI=j19690001161 |journal=Journal of the Chemical Society A: Inorganic, Physical, Theoretical |language=en |pages=1161 |doi=10.1039/j19690001161 |issn=0022-4944}} 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.{{r|Burdett}}
References
External links
- Landskrone Kai. LibreTexts - Chemistry. [https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Inorganic_Coordination_Chemistry_(Landskron)/02%3A_Symmetry_and_Group_Theory/2.02%3A_Point_Groups Section 2.2: Point Groups]
{{MolecularGeometry}}