title: Endohedral and exohedral metalloborospherenes: M@B40 (M=Ca, Sr) and M&B40 (M=Be, Mg).
abstract: The recent discovery of the all-boron fullerenes or borospherenes, D(2d) B40(-/0), paves the way for borospherene chemistry. Here we report a density functional theory study on the viability of metalloborospherenes: endohedral M@B40 (M=Ca, Sr) and exohedral M&B40 (M=Be, Mg). Extensive global structural searches indicate that Ca@B40 (1, C(2v), (1)A1) and Sr@B40 (3, D(2d), (1)A1) possess almost perfect endohedral borospherene structures with a metal atom at the center, while Be&B40 (5, C(s), (1)A') and Mg&B40 (7, C(s), (1)A') favor exohedral borospherene geometries with a η(7)-M atom face-capping a heptagon on the waist. Metalloborospherenes provide indirect evidence for the robustness of the borospherene structural motif. The metalloborospherenes are characterized as charge-transfer complexes (M(2+)B40(2-)), where an alkaline earth metal atom donates two electrons to the B40 cage. The high stability of endohedral Ca@B40 (1) and Sr@B40 (3) is due to the match in size between the host cage and the dopant. Bonding analyses indicate that all 122 valence electrons in the systems are delocalized as σ or π bonds, being distributed evenly on the cage surface, akin to the D(2d) B40 borospherene.
authors.affiliation: Nanocluster Laboratory, Institute of Molecular Science, Shanxi University, Taiyuan 030006 (China).
authors.forename: Hui
authors.initials: H
authors.lastname: Bai
authors.forename: Qiang
authors.initials: Q
authors.lastname: Chen
authors.forename: Hua-Jin
authors.initials: HJ
authors.lastname: Zhai
authors.forename: Si-Dian
authors.initials: SD
authors.lastname: Li
keywords: alloy boron clusters; borospherenes; density functional theory; electron delocalization; metalloborospherenes