Tetracarbon Metallacarboranes 10 On the Thermodynamically Favored Geometry of Large Nido Cages. Structure of the Thermal Rearrangement Product (Isomer III) of (n5-C5H5)CO(CH3)4C4B7H7.

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-C5H5FeCH34C4B7H8 and eta 5-C5H52CO2C4B6H10, isomer VII however, the structures of the three isomers of eta 5-C5H5COCH34C4B7H7 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 C2c, Z 8 a 25.9448, b 8.6863, c 15.4105 A beta 108.473 deg. V 3294 A3 R 0.052 for 1542 independent reflections having F sub 0 squared 3 sigma F sub 0 square. Author