Square antiprismatic molecular geometry

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.

• The distorted square antiprismatic [TaF₈]³⁻ anion in the Na₃TaF₈ lattice.
• The square antiprismatic [XeF₈]²⁻ anion in the lattice of nitrosonium octafluoroxenate(VI), (NO)₂XeF₈.
• Structure of the Bi₈²⁺ cluster in the [Bi₈](GaCl₄)₂.