Gas‐Phase Reaction of Tetraborane(10) with Allene: The Fluxional arachno ‐1‐Carbapentaborane(10) Isomeric System and Derivatives 1,2‐ and 1,3‐Me 2 ‐1‐CB 4 H 8 ; Analogies in 1‐CB 4 H 10 , MeB 5 H 10 , and B 5 H 10 −

An interconverting mixture of isomers , arachno ‐1,3‐ ( 1a ) and arachno ‐1,2‐Me 2 ‐1‐CB 4 H 8 ( 1b ), has been isolated from the quenched gas‐phase reaction of B 4 H 10 with allene and characterized by the ab initio/IGLO/NMR method. The mechanisms of interchange between 1a and 1b , as well as the degenerate isomerizations in the parent compound 1‐CB 4 H 10 and the isoelectronic B 5 H   10 − , were clarified computationally.