The arachno-[B6H11](-) solution structure at -25 degrees C was clarified as fluxional compound 2 by applying the ab initio/IGLO/NMR method. The anion 2 can be derived from arachno-B6H12, 1, by the removal of the B2/B3 bridging hydrogen (2). No minimum on the potential energy surface could be found for an asymmetric complex, a, between [B5H8](-) and BH3, which had been proposed originally. A C-s-symmetric [mu-(BH3)B5H8-](-) complex, A, only 3.2 kcal mol(-1) higher in energy than 2, is the intermediate in the fluxional rearrangement observed on the NMR time scale. The transition structure [D] connecting 2 (E-rel = 0.0) and A (E-rel = 3.2) has a relative energy of 9.7 kcal mol(-1). The elimination of both a and A as "most stable structure" candidates of arachno-[B6H11](-)reinforces the early geometrical bonding systematics for boranes and carboranes.