Tetracarbon Metallacarboranes 10

Alternative Title

On the thermodynamically favored geometry of large nido cages. Structure of the thermal rearrangement product (isomer III) of (η⁵-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 seven 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 (η5-C5H5)Fe(CH3) 4C4B7H8 and (η5-C5H5)2Co2C 4B6H10 (isomer VII); however, the structures of the three isomers of (η5-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 seven established classes of 12-vertex, 28-electron cage systems are discussed in light of the present study. Crystal data are as follows: mol wt 314.9, space group C2/c, Z = 8; a = 25.944 (8), b = 8.686 (3), c = 15.410 (5) Å; β = 108.47 (3)°; V = 3294 Å3; R = 0.052 for 1542 independent reflections having Fo 2 > 3σ(Fo 2). © 1981 American Chemical Society.



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© 1981 American Chemical Society (ACS), All rights reserved.