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.^[1] 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.^[2][3]

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.^[4]

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  The square antiprismatic [XeF₈]²⁻ anion in the lattice of nitrosonium octafluoroxenate(VI), (NO)₂XeF₈.^[5]

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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.^[6] 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.^[2]

References

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