The borderline between classical and nonclassical bonding becomes particularly evident in carbon-rich carbaboranes,"] for example in the series of frameworks nido-C4B2 ,[' I nido-C,B, and nido-C,B, .I4' -dl In general, the boron atoms prefer a nonclassical environment with a high order ofconnectivity, whereas the carbon atoms aim for lower connectivity, ideally with a coordination number of four.[lbl In the case of nido-C4B, and nido-C,B, examples are known for valence isomers with classiand nonclassical structures.['"k,4a-dl Previous attempts at synthesizing tetracarba-nido-octaborane(8) derivatives met limited success with regard to a convenient, straightforward route and to a firm proposal of their molecular struct ~r e . ' ~] Only peralkylated or C-alkylated species were reported.[31 However, all evidence points to a choice between the nonclassical structures A or B; A, which has a hexagonal open face, appears to be energetically favored." b1
We have recently shown that hydroboration of 1 -alkynylsilanes'" or -boranes,[6z 71 carried out under controlled conditions, leads to novel carbaboranes. Here we report that the reaction of bis(diethylbory1)ethyne (1) with 'cdicthylborane"[81 in diethylborane as solvent ("hydridc affords Btetraethyl-tetracarba-nidwoctaborane(8) (2) as a colorless liquid that can be purified by fractional distillation. As shown in Scheme 1, step a, hydroboration of 1 leads to 1,1,2,2-tetrakis(di-cthy1boryl)ethane (3) and/or the 1,1,1.2-isomer 4, which may