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<sub>4d</sub> 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 Na<sub>3</sub>TaF<sub>8</sub>. In this case, with the small Na<sup>+</sup> ions, lattice forces are strong. With the diatomic cation NO<sup>+</sup>, the lattice forces are weaker, such as in (NO)<sub>2</sub>XeF<sub>8</sub>, which crystallizes with a more idealized square antiprismatic geometry.
Square prismatic geometry (D<sub>4h</sub>) 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), [PaF<sub>8</sub>]<sup>3âÂÂ</sup>, as found in its sodium salt, Na<sub>3</sub>PaF<sub>8</sub>. While local cubic 8-coordination is common in ionic lattices (e.g., Ca<sup>2+</sup> in CaF<sub>2</sub>), 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.