In
chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...
, the square antiprismatic molecular geometry describes the shape of
compounds where eight
atom
Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
s, groups of atoms, or
ligand
In coordination chemistry, a ligand is an ion or molecule with a 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 el ...
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 number, even-numbered sequence of triangle sides closed by two polygon caps. It is also known as an ''anticube''.
If all its faces are regular ...
.
This shape has D
4d symmetry
Symmetry () in everyday life refers to a sense of harmonious and beautiful proportion and balance. In mathematics, the term has a more precise definition and is usually used to refer to an object that is Invariant (mathematics), invariant und ...
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. The lanthanide and actinid ...
complexes, along with the
dodecahedron
In geometry, a dodecahedron (; ) or duodecahedron is any polyhedron with twelve flat faces. The most familiar dodecahedron is the regular dodecahedron with regular pentagons as faces, which is a Platonic solid. There are also three Kepler–Po ...
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 convent ...
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 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), (NO)2XeF8.
Examples
*
*
*
Square prismatic geometry and cubic geometry
Square prismatic geometry (D
4h) 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),
aF8sup>3–, as found in its sodium salt, Na
3PaF
8.
While local cubic 8-coordination is common in ionic lattices (e.g., Ca
2+ in CaF
2), 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.
References
External links
*Landskrone Kai. LibreTexts - Chemistry
Section 2.2: Point Groups
{{MolecularGeometry
Stereochemistry
Molecular geometry