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Concept
Square antiprismatic molecular geometry
Summary
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. 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.
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}}
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 Bi82+ cluster in the [[Bismuth polycations|Bi82.]]
XeF82−
IF8−
ReF8−
Square prismatic geometry (D4h) is much less common compared to the square antiprism.
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