The synthesis of a new endohedral ten-vertex Zintl ion cluster, [Fe@Sn10]3−, isoelectronic with [Fe@Ge10]3−, is reported. In an attempt to place this new cluster within the context of the known structural chemistry of the M@E10 family (M = transition metal, E = main group element), we have carried out a detailed electronic structure analysis of the different structural types: viz bicapped square antiprismatic ([Ni@Pb10]2−, [Zn@In10]8−), tetra-capped trigonal prismatic ([Ni@In10]10−) and the remarkable pentagonal prismatic [Fe@Ge10]3− and [Co@Ge10]3−. We establish that the structural trends can be interpreted in terms of a continuum of effective electron counts at the E10 cage, ranging from electron deficient (<4n + 2) in [Ni@In10]10− to highly electron rich (>4n + 2) in [Fe@Ge10]3−. The effective electron count differs from the total valence electron count in that it factors in the increasingly active role of the metal d electrons towards the left of the transition series. The preference for a pentagonal prismatic geometry in [Fe@Ge10]3− emerges as a natural consequence of backbonding to the cage from four orthogonal 3d orbitals of the low-valent metal ion. Our calculations suggest that the new [Fe@Sn10]3− cluster should also exhibit structural consequences of backbonding from the metal to the cage, albeit to a less extreme degree than in its Ge analogue. The global minimum lies on a very flat surface connecting D4d, C2v and C3v-symmetric minima, suggesting a very plastic structure that may be easily deformed by the surrounding crystal environment. If so, then this provides a new and quite distinct structural type for the M@E10 family.