Tetracarbon metallacarboranes. 10. Thermodynamically favored geometry of large nido cages. Structure of the thermal rearrangement product (isomer III) of (.eta.5-C5H5)Co(CH3)4C4B7H7

Abstract

Abstract : The preferred structure of 12-vertex, 28-electron (nido) polyhedral cages, which contain two skeletal electrons beyond the 26 required for a regular closo icosahedral system, and which have thus far been found in at least 7 different structural classes, was the focus of this investigation. The crystal and molecular structure of the title compound, which was formed by thermal rearrangement of isomer I at 140 C, were determined by single-crystal X-ray diffraction. Isomer III has an open cage geometry in which all four carbon atoms and two borons reside on a 6-membered open face, with one of the carbons isolated from the other three. The cage is isostructural with the previously characterized metallacarboranes (eta 5-C5H5)Fe(CH3)4C4B7H8 and (eta 5-C5H5)2CO2C4B6H10, isomer VII; however, the structures of the three isomers of (eta 5-C5H5)CO(CH3)4C4B7H7 are grossly different from each other. The observed geometry of isomer III implies a thermodynamic preference of skeletal carbon atoms for low-coordinate vertices on the open rim, even though this requires three of the four carbons to remain adjacent. The adoption of a high-coordinate vertex by cobalt is also significant. The 7 established classes of 12-vertex, 28-electron cage systems are discussed in light of the present study. Crystal data: mol wt 314.9, space group C2/c, Z = 8; a = 25.944(8), b = 8.686(3), c = 15.410(5) A; beta = 108.47(3) deg.; V = 3294 A3; R = 0.052 for 1542 independent reflections having F sub 0 squared 3 sigma (F sub 0) square. (Author)