The 12-vertex endohedral cluster [Ru@Ge12](3-) reveals an unprecedented D2d-symmetric 3-connected polyhedral geometry. The structure contrasts dramatically with the known deltahedral or approximately deltahedral geometries of [M@Pb12](2-) (M = Ni, Pd, Pt) and [Mn@Pb12](3-) and is a result of extensive delocalization of electron density from the transition-metal center onto the cage.

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.

Synthesis and Characterization of [Ru@Ge 12] 3-: An Endohedral 3-Connected Cluster. -Compound (III) crystallizes in the monoclinic space group P2 1 with Z = 2. The 12-vertex endohedral cluster [Ru@Ge 12] 3exhibits an unprecedented D2d-symmetric 3-connected polyhedral geometry. The cluster is further characterized by DFT calculations. -(ESPINOZA-QUINTERO, G.; DUCKWORTH, J. C. A.; MYERS, W. K.; MCGRADY*, J. E.; GOICOECHEA, J. M.; J. Am. Chem. Soc. 136 (2014) 4, 1210-1213, http://dx.