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.

• 
  The distorted square antiprismatic [TaF₈³⁻ anion in the Na₃TaF₈ lattice. ^[4]
• 
  The square antiprismatic [XeF₈²⁻ anion in the lattice of nitrosonium octafluoroxenate(VI), (NO)₂XeF₈. ^[5]
• 
  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