Reaction of 2 moles of tropenylium ion with B10H102- or B12H122- yields tropenyliumyl-closo-nonahydro-decaborate(1 –) or tropenyliumyl-closo-undecahydrododecaborate(1 –) ion and cycloheptatriene; these highly colored anions form salts with considerable chemical and thermal stability. Infrared, electronic, and 1H and 11B nmr spectra, Z-value correlations, and polarographic reduction potentials show that in both ions a cationic carbocyclic ring is attached to the boron cage with a C-B bond, and that significant cage-to-ring electron donation occurs in both ground and excited states. © 1969, American Chemical Society. All rights reserved.