A Stannyl-Decorated Zintl Ion [Ge₁₈Pd₃(SnⁱPr₃)₆]^2–: Twinned Icosahedron with a Common Pd₃-Face or 18-Vertex Hypho-Deltahedron with a Pd₃-Triangle Inside

We report the synthesis and characterization of the title anion which has a germanium/palladium cluster core of [Ge18Pd3] and six tri-isopropyl tin substituents. Its two Ge9-halves are the first examples of germanium deltahedra with three nonsilyl substituents, tri-isopropyl tin in this case. The new cluster is made by a reaction of an acetonitrile suspension of K4Ge9 with (i)Pr3SnCl that generates primarily tristannylated 9-atom clusters [Ge9{Sn(i)Pr3}3](-), followed by addition of Pd(PPh3)4 to the reaction mixture. It was structurally characterized by single-crystal X-ray diffraction in [K(222crypt)]2[Ge18Pd3{Sn(i)Pr3}6]·(i)Pr2O and was also confirmed in solution by ESI-MS and NMR. The new anion can be viewed both as a dimer of face-fused icosahedra (twinned icosahedron) with a common Pd3-face, i.e., [((i)Pr3Sn)3Ge9Pd3Ge9(Sn(i)Pr3)3](2-) that resembles but is not isoelectronic with the known borane version B21H18(-) or as a large hypho-deltahedron of 18 Ge-atoms with a triangle of Pd3 inside, i.e., [Pd3@Ge18(Sn(i)Pr3)6](2-). DFT calculations show a very large HOMO-LUMO gap of 2.42 eV.