9-Br-closo-1,2-C2 B10 H11 has been prepared and its gas-phase structure has been examined by means of gas electron diffraction. The structure of the carbaborane core is similar to the structure of the parent compound, which is of C2v symmetry. A DFT-based search for the corresponding reaction pathway of the bromination of closo-1,2-C2 B10 H12 revealed that the catalytic amount of aluminum reduces the barrier of the initial attack of the bromination agent toward the negatively charged part of the icosahedral carbaborane, i.e., the first transition state, from about 40 to about 27 kcalmol-1 . The Br-Br bond is weakened by an intermediate binding to the large pi-hole on the aluminum atom of AlBr3 , which is the driving force for the AlBr3 -catalyzed bromination. © 2020 Wiley-VCH GmbH.