Triple-decker and tetradecker sandwich complexes having planar C2B3 or pyramidal C2B4 carborane ligands at one or both ends, the first examples of this genre, were prepared and characterized. A 3-way reaction of the Cp*Co(2,3-Et(2)C(2)B(3)H(2)-5-Me)(2-) and 2,3-Et(2)C(2)B(4)H(4)(2-) dianions with CoCl2 gave the diamagnetic 30 ve (valence electron) triple-decker Cp*Co(Et(2)C(2)B(3)H(2)Me)CoH(Et(2)C(2)B(4)H(4)) (2) and the paramagnetic 29 ve triple-decker Cp*Co(Et(2)C(2)B(3)H(2)Me)Co(Et(2)C(2)B(4)H(4)) (3) Decapitation of these species gave diamagnetic Cp*Co(Et(2)C(2)B(3)H(2)Me)CoH(Et(2)C(2)B(3)H(5)) (4), whose triple-decker geometry was confirmed by an X-ray structure determination that established the location of the hydrogen atom capping a CoB2 triangular face. Removal of the CoH proton in 4 followed by oxidation of the anion in air gave paramagnetic Cp*Co(Et(2)B(3)H(2)Me)Co(Et(2)C(2)B(3)H(5)) (5) Treatment of neutral 4 and 5 with n-bromosuccinimide afforded the mono- and dibromo derivatives 6-8. The 30 ve triple-decker Cp*Co(Et(2)C(2)B(3)Me(3))Ni(Et(2)C(2)B(4)H(4)?) (10) was prepared from the peralkylated synthon Cp*Co(2,3-Et(2)C(2)B(3)Me(3))(2-), Et(2)C(2)B(4)H(4)(2-), and NiBr2. A similar approach employing the cymeneruthenacarborane dianion (CHMe(2)C(6)H(4)Me)Ru(Et(2)C(2)B(3)H(2)Me)(2-) gave the paramagnetic triple-decker (CHMe(2)C(6)H(4)Me)Ru(Et(2)C(2)B(3)H(2)Me)Co(Et(2)C(2)B(4)H(4)) (12), which was decapitated to yield diamagnetic (CHMe(2)C(6)H(4)Me)Ru(Et(2)C(2)B(3)H(2)Me)CoH(Et(2)C(2)B(3)H(5)) (13); the latter species was air-oxidized to afford 29 ve (CHMe(2)C(6)H(4)Me)Ru(Et(2)C(2)B(3)H(2)Me)Co(Et(2)C(2)B(3)H(5)) (14). Stacking reactions of the nido, close dianion (Et(2)C(2)B(3)H(4)Me)Co(Et(2)C(2)B(4)H(4))(2-) (15(2-)) with CO2+ or Ni2+ ions gave the paramagnetic carborane-bicapped tetradecker sandwiches M[(Et(2)C(2)B(3)H(2)Me)Co(Et(2)C(2)B(4)H(4))](2) (16, M = CO. 41 ve; 17, M = NiH, 43 ve). Reactions of the 15(3-) trianion with Cp*Co(2,3-Et(2)C(2)B(3)H(2)Me)(2-) ion and Co2+ or Ni2+ gave the carborane-monocapped tetradeckers Cp*Co(Et(2)C(2)B(3)H(2)Me)M(Et(2)C(2)B(3)H(2)Me)COH(Et(2)C(2)B(4)H(4) (18, M = Co, 41 ve; 19, M = Ni, 42 ve, diamagnetic). Decapitation of 18 gave paramagnetic 41 ve Cp*Co(Et(2)C(2)B(3)H(2)Me)Co(Et(2)C(2)B(3)H(2)Me)CoH(Et(2)C(2)B(3)H(5)) (20), a 41 ve tetradecker sandwich having an open carborane end ligand. The new compounds were characterized via UV-visible and mass spectra, H-1 and B-11 NMR spectra (for diamagnetic species), and ESR spectra (for paramagnetic complexes). Features of the spectroscopic data are discussed and related to the electronic structures of these systems. Crystal data for 4 : space group P2(1)/a; a = 9.312(3) Angstrom, b = 28.25(1) Angstrom, 10.234(4) Angstrom, beta = 95.56(3)degrees; Z = 4; R = 0.048 for 3834 independent reflections having I > 3 sigma(I).磁S