In chemistry, the square antiprismatic molecular geometry describes the shape of compounds where eight

atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, ...

s, groups of atoms, or

ligand In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elec ...

s are arranged around a central atom, defining the vertices of a

square antiprism In geometry, the square antiprism is the second in an infinite family of antiprisms formed by an even-numbered sequence of triangle sides closed by two polygon caps. It is also known as an ''anticube''. If all its faces are regular, it is a sem ...

. This shape has D_4d symmetry and is one of the three common shapes for octacoordinate

transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that ca ...

complexes, along with the

dodecahedron In geometry, a dodecahedron (Greek , from ''dōdeka'' "twelve" + ''hédra'' "base", "seat" or "face") or duodecahedron is any polyhedron with twelve flat faces. The most familiar dodecahedron is the regular dodecahedron with regular pentagon ...

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⁺

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

s, 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 aF₈sup>3− anion in the Na₃TaF₈ lattice. File:Octafluoroxenate(VI)-3D-balls-A.png, The square antiprismatic eF₈sup>2− anion in the lattice of

nitrosonium octafluoroxenate(VI) Nitrosonium octafluoroxenate(VI) is a chemical compound of xenon with nitrogen, oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly r ...

, (NO)₂XeF₈. File:EntryWithCollCode414090.png, Structure of the Bi₈²⁺ cluster in the i₈GaCl₄)₂.

Examples

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References

{{MolecularGeometry Stereochemistry Molecular geometry