Experimental and theoretical studies on intermetalloid cluster anions, [1] that is, main-group-element cages with interstitial transition metal atom(s), have attracted the interest of chemists and physicists for about a decade. This is due to a large variety of novel structural types, unprecedented bonding situations, and unexpected chemical and physical properties of the resulting phases; furthermore, the compounds are discussed as being models for doped materials and/or precursors to novel intermetallic phases.[2] The structures of the known intermetalloid anions are controlled by both the synthetic reaction route and the embedded transition metal atom. , [4] that would not exist without interstitial atom. These anions feature exclusively nondeltahedra faces and thus direct toward fullerene-like molecular structures.[2a]To date, it has not been possible to isolate main-groupmetal cages stabilized by Ln ions, although these elements are well-known as both components of intermetallic phases, for example, in EuSn 3 Sb 4 , [5] and as guests in doped main-groupelement host lattices, for example in nanocrystalline LED phosphors such as M 2 Si 5 N 8 :Eu 2+ (M = Sr, Ba) [6] or laser materials such as Nd:YAG.[7] Moreover, photoelectron spectroscopy [8] and quantum chemical investigations [9] indicated the existence of [LnSi n ] À species (3 n 13; Ln = Ho, Gd, Pr, Sm, Eu, Yb), [EuSi n ] À (3 n 17), and Yb@Pb 12 in the gas phase. In the condensed phase, Ln ions have only been trapped by carbon cages such as M@C 82 and M 2 @C x (M = LaNd, Sm-Lu; x = 72, 78, 80).[10] These clusters have been discussed as "designer" materials because a small variation in their composition can significantly manipulate their chemical, electronic, and magnetic properties. For instance, the magnetic moment is tunable by the type and oxidation state of the incorporated lanthanide ion. [8] We (Figure 1) form a polyhedron that consists of nine nondeltahedral faces, namely six pentagons and three square faces. This enneahedron has been unknown to date in an isolated, ligand-free form. However, topologically identical polyhedra were previously observed and discussed as part of complicated networks within two solid-state phases: elongated in the intermetallic phase Ag 7 Te 4[13] or undistorted in