metal clusters, or in constrained coordination environments such as condensed phases or the hexagonal pores of coordination polymers. The interconversion of six-coordinate geometries can be considered by inspecting minimal distortion pathways. An octahedron can be viewed as two tetrahedrons sharing a vertex. Rotation of one tetrahedron around a C3 axis results in distortion from an octahedral to a trigonal prismatic geometry. The angle associated with this rotation is often referred to as the twist angle (q). An octahedron has a value of q = 60° while the trigonal prism has a value of q = 0°. Interconversion by rotation (Bailar twist) is a wellestablished phenomenon in transition metal chemistry. A similar analysis can be conducted for the theoretical interconversion between octahedral and hexagonal planar geometries. Symmetric flattening of the tetrahedra will result in the formation of a hexagon. A planarization angle (f) can be defined; an octahedron has a value of f = sin -1 (1/Ö3), which corresponds to 35.3°. When f = 0° a hexagonal planar geometry is obtained. Further, variation of q from 60° to ~ 30° while keeping f = 0° interconverts the hexagonal planar and trigonal planar geometries. This operation results in compression of the ligand---ligand distances and hence q cannot reach 0° as it would result in the superposition of the ligand atoms. This distortion formally interconverts a six-coordinate and three-coordinate geometry, but it is only possible for systems in which the ligands can form bonds with each other at the expense of metal-ligand bonding (Figure ). A bonding continuum between the hexagonal planar and trigonal planar geometries can, therefore, be considered.