Edge

In the edge isomer the empty edges (red, red, pink) share a common (red, red) edge and are shown in heavy outline. The four equivalent methyl groups over the (red, pink, blue) faces make up a rectangle. The symmetry constraints prevent the methyl groups relaxing into a lower energy geometry such as is taken on in the trans or apical isomer. This results in the edge having the highest energy of the three isomers.