One of the most significant recent developments (in 2009) is the discovery of the clusters M@Ge-10(3-) (M = Fe, Co) in which the outer Ge-10 polyhedron is a pentagonal prism rather than a deltahedral structure of the type predicted by the Wade-Mingos rules. Consistent with this experimental observation, density functional theory shows the lowest energy structures to be pentagonal prisms for the iron-centered clusters Fe@Ge-10(z) in all nine charge states ranging from -5 to +3. This contrasts with the previously studied cobalt-centered germanium clusters Co@Ge-10(z) for which the lowest energy structures are pentagonal prisms only for the electron richest systems where z ranges from -3 to -5. The C-3v structures derived from the tetracapped trigonal prism found as lowest energy structures of the electron poorer Co@Ge-10(z) (z = 0, -1, -2) systems are higher energy structures for the iron-centered germanium clusters Fe@Ge-10(z) (z = 0, -1, -2). The strong energetic preference for pentagonal prismatic structures in the Fe@Ge-10(z) clusters can be attributed to the need for the larger volume of the pentagonal prism relative to other 10-vertex closed polyhedra to accommodate the interstitial iron atom.