The formation of icosahedral carboranes, their degradation to dicarbollide ions, and formation of metallocarboranes from these ions are briefly reviewed. The bis(dicarbollyl) cobalt(ni) anion reacts with cobalt hydroxide to produce bi-and tn-metallic polyhedral complexes incorporating the (3 ó)-l.2-B8C2H ion and possessing novel structures. The strong acid (3)-1,2-B9C2H" reacts with R3M(R alkyl, M = Al or Ga) in two steps, first to give a dialkylmetalsubstituted carborane, ( 3 )-1,2-B9C2H1 2M

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... . X-Ray diffraction and variable temperature n.m.r. (250 MHz 'H and 80 MHz "B) show this carborane is bonded by B-H-Al bridge bonds and undergoes rapid tautomeric rearrangement at room temperature. It reacts further to form a monoalkylmetalsubstituted carborane, (3)-1,2-B9C2H11MR. This compound rearranges at 400°C to afford (3)-1,7-B9C2H"MR. Thermal rearrangement also occurs in (ir-C5H,) Co[ir-(3)-1,2-B9C2H"] ; six new isomers which result from migration of carbon atoms over the polyhedron have been isolated and characterized. Products from the thermal rearrangement of (ir-C5H5)Co[ir-(3)-1,2-B9C2H9 (CH,)2] and (it-C5H5)Co[it-(3)-1,2-B9C2119(CI-12)3] are also reported. Structural elucidation and mechanistic implications of these rearrangements are discussed. Several closed polyhedral carboranes undergo an apparent reductive opening to afford dianions which, upon subsequent reaction with CoCl2 and C5H, form new closed polyhedra which incorporate the metal; these derivatives have the overall geometry of the next highest homologue of the known polyhedral species. Complexes formed in this manner from 1,6-B6C2H8, 1,6-B8C2H,0, and 12-B,0C2H12 are reported. The red 13-atom polyhedral complex (it-C5H 5)Co(7,8 ,0C2H 2) undergoes thermal rearrangement to two new isomers, orange and red-orange, which probably differ only in positions of the carbons in the polyhedron.