CRYSTAL-STRUCTURE OF A TETRACOBALT TETRABORON CLUSTER, (ETA-5-C5H5)4CO4B4H4 - STRUCTURAL PATTERNS IN 8-VERTEX POLYHEDRA

The structure of the title compound was determined by single-crystal X-ray diffraction and found to consist of an eight-vertex C04B4 closo polyhedron with symmetry very close to idealized Did. The cobalt atoms occupy contiguous positions at the 5-coordinate vertices on the polyhedron while the boron atoms are located at the four 4-coordinate vertices, as orginally proposed from boron-11 and proton NMR data. The compound crystallizes in the monoclinic C2/c space group. Of the 12 molecules in the unit cell, eight are in general positions while the remaining four are bisected by crystallographic twofold axes. The molecule in a general position was found to have a 0.20-weight chlorine atom bonded to one of the borons, evidently as a consequence of repeated recrystallization from chlorinated hydrocarbon solvents. The violation of Wade's electron-counting rules by the Co4B4 cluster, which has only 16 skeletal electrons in comparison with 18 normally expected for an eight-vertex closo polyhedron, is discussed in light of the established structures of other eight-vertex cluster compounds. Crystal data: M, = 543.4; space group C2/c; a = 23.70 (2), b = 17.974 (7), c = 18.562 (10) A; 0 = 129.14 (5)°; V= 6134 (7) A3; n(Mo Ka) = 33.4 cm-1; pcalcd(for Z = 12) = 1.780 g cm-3. The structure was refined by full-matrix least-squares methods to a final R value of 0.042 for the 1912 reflections for which F02 > 3<r(F02). © 1979, American Chemical Society. All rights reserved.