exo ‐ endo Isomerism of Carboranes: Unusual Geometries of C ₂ B ₅ X ₇ ( X =Cl, Br) and C ₂ B ₇ Cl ₉ with exo ‐Skeletal BCl ₂ Groups on Carbon Revealed by Joint Spectroscopic/Computational Studies

Reexamination of the co‐pyrolysis reactions of B 2 Cl 4 with C 2 Cl 4 at 350 °C and of B 2 Br 4 with CBr 4 at 300 °C in vacuo confirmed the carboranes C 2 B 5 Cl 7 ( 1 ), C 2 B 7 Cl 9 ( 2 ), and C 2 B 5 Br 7 ( 3 ) as low‐yield products. While 1 only could be concentrated by repeated vacuum fractionation, 2 and 3 could now be isolated from the conglomerate mixtures for a full spectroscopic characterization and the compounds were verified in their geometries by detailed DFT computations. Surprisingly, the perhalogenated carboranes do not adopt the expected “all‐ endo ”‐geometries with cluster sizes derived by the sum of the n boron and two carbon atoms ( n+2 ) as known from the syntheses of the parent closo ‐carboranes C 2 B n H n+2 . Instead, DFT/GIAO(ZORA)/NMR (GIAO for X =Cl, ZORA for X =Br) computations revealed that the perhalogenated carboranes favor structures with B X 2 groups as exo ‐skeletal ligands attached to both cage‐carbon atoms yielding the five‐vertex closo ‐1,5‐(CB X 2 ) 2 B 3 X 3 ( 1 : X =Cl; 3 : X =Br) and the seven‐vertex closo ‐2,4‐(CBCl 2 ) 2 B 5 Cl 5 for 2 . In contrast to these perhalogenated carboranes, analogous computations on the hydrogen substituted carboranes C 2 B n H n+2 , silaboranes Si 2 B n H n+2 and Si 2 B n X n+2 ( n =5, 7) show in all cases a thermodynamic favorization of structures where all boron atoms of the formula are endo ‐skeletally incorporated into the cluster frameworks.