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 Na
3TaF
8. 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)
2XeF
8, which crystallizes with a more idealized square antiprismatic geometry.
File:TaF83-Core.png, The distorted square antiprismatic 8">aF8sup>3− anion in the Na3TaF8 lattice.
File:Octafluoroxenate(VI)-3D-balls-A.png, The square antiprismatic 8">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 8GaCl4)2.">i8GaCl4)2.
Examples
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References
{{MolecularGeometry
Stereochemistry
Molecular geometry