The structures of a tetracarba-nido-octaborane(8) and a novel spiro derivative of a 2,3,5-tricarba-nido-hexaborane(7)

Hydroboration of bis(diethylboryl) ethyne (1) with tetraethyldiborane(6) le ads to a B-ethyl-substituted tetracarba-nido-octaborane 2, a spiro-carboran e 3, which belongs to the 2,3,5-tricarba-nido-carborane family, and a hexac arba-arachno-dodecaborane(12) 4, along with polymeric material. The X-ray s tructure analysis of carborane 2, determined here, is fully consistent with the structure deduced earlier from NMR data. The structure of 3 in solutio n was established by theoretical analysis of its NMR data. Ab initio calcul ations of the structures of 2b and 3b (b denotes the B-metyhl-substituted d erivatives) and the comparison of calculated with experimental NMR data sup port the suggested structures of 2 and 3 in solution. The calculated struct ure of the carborane cage of 2a also agrees with the experimental geometry of 2.