Syntheses, properties, and synthetic applications of 13-vertex closo-and nido-carboranes are reported. Reactions of the nido-carborane salt [(CH2)(3)C2B10H10]Na-2 with dihaloborane reagents afforded 13-vertex closo-carboranes 1,2-(CH2)(3)-3-R-1,2-C2B11H10 (R = H (2), Ph (3), Z-EtCH=C(Et) (4), E-'BuCH= CH (5)). Treatment of the arachno-carborane salt [(CH2)(3)C2B10H10]Li-4 with HBBr2 center dot SMe2 gave both the 13-vertex carborane 2 and a 14-vertex closo-carborane (CH2)(3)C2B12H12 (8)- On the other hand, the reaction Of (C6H4(CH2)(2)C2B10H10]Li-4 with HBBr2 center dot SMe2 generated only a 13-vertex c/oso-carborane 1,2-C6H4(CH2)(2)-1,2-C-2 B11H11 (9). Electrophilic substitution reactions of 2 with excess Mel, Br-2, or 12 in the presence of a catalytic amount of AlCl3 produced the hexa-substituted 13-vertex carboranes 8,9,10,11,12,13-X-6-1,2-(CH2)(3-)1,2-C2B11H5 (X = Me (10), Br (11), 1 (12)). The halogenated products 11 and 12 displayed unexpected instability toward moisture. The 13-vertex closo-carboranes were readily reduced by groups 1 and 2 metals. Accordingly, several 13-vertex nido-carborane dianionic salts {nido-1,2-(CH2)(3)-1,2-C2B11H11}{Li-2(DME)(2-)(THF)(2)} (13), [{nido-1,2-(CH2)(3)-1,2-C2B11H11}Na-2(THF)(4)}](n) (13a), [{nido-1,2-(CH2)(3)-3-Ph-1,2-C2B11H10}-{Na-2(THF)(4)}](n) (14), [{nido-1,2-C6H4(CH2)(2)-1,2-C2B11H11}Na-2(THF)(4)}](n) (15), and {nido-1,2-(CH2)(3)-1,2-C2B11H11}{M(THF)(5)} (M = Mg (16), Ca (17)) were prepared in good yields. These carbon-atom-adjacent nido-carboranes were not further reduced to the corresponding arachno species by lithium metal. On the other hand, like other nido-carborane dianions, they were useful synthons for the production of super-carboranes and supra-icosahedral metallacarboranes. Interactions of 13a with HBBr2 center dot SMe2, (dppe)NiCl2, and (dppen)NiCl2 gave the 14-vertex carborane 8 and nickelacarboranes [eta(5)-(CH2)(3)C2B11H11]Ni(dppe) (18) and [eta(5)-(CH2)(3)C2B11H11]Ni(dppen) (19), respectively. All complexes were fully characterized by various spectroscopic techniques and elemental analyses. Some were further confirmed by single-crystal X-ray diffraction studies.