The octahedral structure is not the only possible six- coordinate structure. other possibilities include a planar hexagonal structure and a triangular prism structure. in the latter, the ligands are arranged in two parallel trian- gles, one lying above the metal atom and the other below the metal atom with its corners directly in line with the corners of the first triangle. show that the existence of two and only two isomers of [co(nh3)4cl2]1 is evidence against both of these possible structures.