Electronic Control of the Position of the Pb Atom on the Surface of B 8 Borozene in the PbB 8 Cluster.

Spontaneous symmetry-breaking is common in chemical and physical systems. Here, we show that by adding an electron to the C 7 v PbB 8 cluster, which consists of a planar B 8 disk with the Pb atom situated along the C 7 axis, the Pb atom spontaneously moves to the off-axis position in the PbB 8 - anion. Photoelectron spectroscopy of PbB 8 - reveals a broad ground-state transition and a large energy gap, suggesting a highly stable closed-shell PbB 8 borozene complex and a significant geometry change upon electron detachment. Quantum chemistry calculations indicate that the lowest unoccupied molecular orbital of the C 7 v PbB 8 cluster is a degenerate π orbital mainly consisting of the Pb 6p x and 6p y atomic orbitals. Occupation of one of the 6p orbitals spontaneously break the C 7 v symmetry in the anion due to the Jahn-Teller effect. The large amplitude of the position change of Pb in PbB 8 - relative to PbB 8 is surprising owing to bonding interactions between the Pb 6p orbital with the π orbital of the B 8 borozene.