Structural variations on an electron precise theme: Rationalising the structures of main group cages

Abstract: The interplay between electron count and molecular geometry is a central theme in main group chemistry. The structures of electron-precise cages, which offer three orbitals and three electrons for skeletal bonding, were rationalised by Mingos based on cage structures comprising n three-coordinate vertices (for n =4, 6, 8, 10, etc.). Whilst these offered a conceptual rationale, an increasing number of potential structural topologies exist with increasing cage size, thereby limiting the predictive nature of this approach (e.g. there are three possible cage topologies for 8 vertex polyhedra and five for 10-vertex polyhedra!). The current review re-examines the structural motifs possible for these electron-precise cages, focusing not only on the cage size (number of vertices) but also on the number and size of the faces of the cage. This approach reveals rather limited numbers of favourable topologies for the large family of hetero-atomic electron-precise cages (AB) x in which electrostatic contributions are maximised for topologies comprising only even-numbered (e.g. square and hexagonal) faces. This review focuses on larger cages (x ≥6) identifying the most favourable geometries for these structures up to 20-vertex molecular cages. Larger cages are illustrated with suitable examples where available. [Copyright &y& Elsevier]