[Nb@Ge 13/14 ] 3- : New Family Members of Ge-Based Intermetalloid Clusters.

During the past two decades, single-atom-centered medium-sized germanium clusters [M@Ge n ] (M=transition metals, n>12) have been extensively explored, both from theoretical perspectives and experimental gas-phase syntheses. However, the actual structural arrangements of the Ge 13 and Ge 14 endohedral cages are still ambiguous and have long remained an unresolved problem for experimental implementation. In this work, we successfully synthesize 13-/14-vertex Ge clusters [Nb@Ge 13 ] 3- (1) and [Nb@Ge 14 ] 3- (2), which are structurally characterized and exhibit unprecedented topologies, neither classical deltahedra nor 3-connected polyhedral structures. Theoretical analysis indicates that the major stabilization of the Ge backbones arises due to the substantial interaction of Ge 4p-AOs with the endohedral Nb 4d-AOs through three/four-center two-electron bonds with an enhanced electron density accumulated over the shortest Nb-Ge13 contact in 1. Low occupancies of the direct two-center two-electron (2c-2e) Nb-Ge and Ge-Ge σ bonds point to a considerable degree of electron delocalization over the Ge cages revealing their electron deficiency.