Treatment of an acidified solution of the [nido-7,8-C2B9H12](-) anion (1(-)) with NaNO2 at 0 degrees C in the presence of benzene resulted in the formation of two eleven-vertex azadicarbaboranes, nido-10,7,8-NC2B8H11 (2) and arachno-1,8,11-NC2B8H13 (3), isolated in yields of 15 and 35%, respectively, together with a small amount (0.9%) of 5-Ph-nido-7,8,10-C2NB8H10 (5-Ph-2). Compound 3 was converted in 68% yield into 2 by reaction with PS (PS = "proton sponge''; 1,8-(dimethylmino)naphthalene and acetone. Deprotonation of 2 at the N(10)H vertex gave the [nido-10,7,8-NC2B8H10](-) anion (2(-)), which was easily alkylated with Me2SO4 or PhCH2Br to produce the N-alkylated derivatives of 2, 10-R-nido-10,7,8-NC2B8H10, where R = Me (10-Me-2, 86%) and PhCH2 (10-PhCh(2)-, 2, 69%). The geometries of the parent dicarbazaboranes 2 and 3 were optimized at the MP2(fc)/6-31G* level, and the structures of all compounds were thence confirmed by the excellent agreement between experimental data and IGLO/NMR calculations of the B-11 chemical shifts for the parent compounds at the DZ//6-31G*, DZ//MP2/6-31G*, and II'//MP2/6-31G* levels.