Physics:Square antiprismatic molecular geometry

Square antiprismatic molecular geometry
Examples	XeF2−8, ReF−8
Point group	D4d
Coordination number	8
μ (Polarity)	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. This shape has D_4d 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.

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  The distorted square antiprismatic [TaF₈]³⁻ anion in the Na₃TaF₈ lattice.

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  The square antiprismatic [XeF₈]²⁻ anion in the lattice of nitrosonium octafluoroxenate(VI), (NO)₂XeF₈.

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  Structure of the Bi₈²⁺ cluster in the [Bi₈](GaCl₄)₂.

Examples

• XeF2−8
• IF−8
• ReF−8

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), [PaF₈]^3–, as found in its sodium salt, Na₃PaF₈. 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.