ALTHOUGH the number of atomic species with co-ordination number eight is not large, little is known about the structure of eight-co-ordinated complexes. Up to now only two of five possible polyhedra have been established for this co-ordination, namely, a trigonal dodecahedron and an Archimedean square antiprism1, while cube, triangular face-centred and rectangular face-centred trigonal prisms have not yet been observed. These polyhedra were proposed on the basis of quantum-mechanical calculations on the form of hybrid bond orbitals.