Electronic Structure and Stability of [B12X12]2- (X = F-At): A Combined Photoelectron Spectroscopic and Theoretical Study.

Abstract

The stability and electron loss process of numerous multiply charged anions (MCAs) have been traditionally explained in terms of the classical Coulomb interaction between spatially separated charged groups. An understanding of these processes in MCAs with not well-separated excess charges is still lacking. We report the surprising properties and physical behavior of [B12X12]2-, X = F, Cl, Br, I, At, which are MCAs with not well-separated excess charges and cannot be described by the prevailing classical picture. In this series of MCAs, comprising a "boron core" surrounded by a "halogen shell", the sign of the total charge in these two regions changes along the halogen series from X = F-At. With the aid of experimental photoelectron spectroscopy and highly correlated ab initio electronic structure calculations, we demonstrate that the trend in the electronic stability of these MCAs is determined by the interplay between the Coulomb (de)stabilization originating from the "boron core" and "halogen shell" and the extension of the overlap between the orbitals from both regions. The second excess electron is always taken from the most positively charged region, viz., the "boron core" for X = F, Cl, and the surrounding "halogen shell" for X = I, At. This change in the physical behavior is attributed to the position of the highest occupied molecular orbital, which dwells in a region that is spatially separated from the one containing the excess negative charge. The unusual intrinsic electronic structure of the [B12X12]2- MCAs provides the basis for a molecular level understanding of their observed unique physical and chemical properties and a new paradigm for understanding the properties of these MCAs with not well-separated charges that departs from the prevailing model used for spatially separated charges that is based on their classical Coulomb interaction.