The 12-vertex endohedral cluster [Ru@Ge-12](3-) reveals an unprecedented D-2d-symmetric 3-connected polyhedral geometry. The structure contrasts dramatically with the known deltahedral or approximately deltahedral geometries of [M@Pb-12](2-) (M = Ni, Pd, Pt) and [Mn@Pb-12](3-) and is a result of extensive delocalization of electron density from the transition-metal center onto the cage.