Single-Metal-Encapsulated Double-Cage [Pt@Sn 17 ] 4- : An Exception from Group 14 Endohedral Clusters.

Group 14 endohedral clusters containing a metal center inside usually possess a single cage topological structure, but here an unexpected single-metal-filled double-cage cluster, [Pt@Sn 17 ] 4- (1 a) is reported. It can be seen as a combination of the more extended Pt-filled [Pt@Sn 9 ] cage and hollow [Sn 9 ] cage sharing a central Sn atom, which is offset from the central position. This double-cage species represents the largest group 14 intermetalloid cluster encapsulating a single transition metal atom. DFT calculations show that the capsule-like architecture of [Sn 17 ] 4- , similar to that found in [Pt 2 @Sn 17 ] 4- , is unstable if filled with a single Pt atom and collapses to the title cluster 1 a upon geometry optimization. Deviation of the central Sn atom occurs due to the vibronic coupling as a consequence of pseudo-Jahn-Teller distortion leading to the bent C s -symmetrical structure, in contrast to the more symmetrical D 2d cage previously reported in [Ni 2 @Sn 17 ] 4- .