Endohedral and Exohedral Metalloborospherenes: M@B 40 (M=Ca, Sr) and M&B 40 (M=Be, Mg)

The recent discovery of the all‐boron fullerenes or borospherenes, D 2d B 40 −/0 , paves the way for borospherene chemistry. Here we report a density functional theory study on the viability of metalloborospherenes: endohedral M@B 40 (M=Ca, Sr) and exohedral M&B 40 (M=Be, Mg). Extensive global structural searches indicate that Ca@B 40 ( 1 , C 2v , 1 A 1 ) and Sr@B 40 ( 3 , D 2d , 1 A 1 ) possess almost perfect endohedral borospherene structures with a metal atom at the center, while Be&B 40 ( 5 , C s , 1 A′) and Mg&B 40 ( 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+ B 40 2− ), where an alkaline earth metal atom donates two electrons to the B 40 cage. The high stability of endohedral Ca@B 40 ( 1 ) and Sr@B 40 ( 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 B 40 borospherene.