TETRACARBON METALLOCARBORANES .8. STRUCTURE OF A 12-VERTEX ARACHNO CARBORANE, SIGMA-(ETA-5-C5H5)CO(ETA-5-C5H4)+-(CH3)4C4B8H8-, AN ANALOG OF B12H126-AND C2B10H124- - MECHANISMS OF CARBORANE FLUXIONAL BEHAVIOR AND METALLOCARBORANE FORMATION FROM THE (CH3)4C4B8H82-ION

The structure of the title compound, the first confirmed example of a 12-vertex arachno borane cage, was determined by single-crystal X-ray diffraction and is formulated as a cobaltocenium-substituted derivative of the (CH3)4C4B8H9-ion, which in turn is obtained by protonation of the (CH3)4C4B8H82-dianion. The carborane polyhedron, a 30-electron, 12-vertex cage, has an open basket-like geometry in which the four C-CH3groups occupy contiguous positions on the open face. One of the C-CH3units is coordinated to only two framework atoms, and the “extra” hydrogen atom is attached to this bridging carbon. The methyl group on the bridging carbon atom is located over the open face of the cage, and the cobaltocenium substituent is attached to a boron atom on the open face adjacent to the bridging carbon. From this structure and the previously determined geometry of neutral (CH3)4C4B8H8, a mechanism is proposed for the formation of the dianion and for its fluxional behavior in solution. Based on this mechanism, a scheme is proposed to account for the formation of several tetracarbon ferracar-boranes from the (CH3)4C4B8H9-dianion. The interconversion of neutral (CH3)4C4B8H8isomers and the probable structure and fluxionality of isomer B are discussed. The σ-(C5H5)Co(C5H4)+-(CH3)4C4B8H8-molecule crystallizes in the triclinic 11 space group with [formula ommitted] for the 7516 reflections for which F02 3 er(F02). There are 12 molecules in the unit cell and three molecules per asymmetric unit. Full-matrix least-squares refinement of the positional and thermal parameters of all 168 atoms (including all hydrogens) in the asymmetric unit gave a final R value of 0.047. © 1979, American Chemical Society. All rights reserved.