Synthesis and structural studies, employing combined NMR, X-ray crystallographic, and ab initio/IGLO/NMR methods, of a variety of new subicosahedral carboranes with adjacent cage carbons are reported. Acetonitrile-induced cage degradation of arachno-4,5-C2B7H12- gave nido-4,5-C2B6H9- (1(-)) in nearly quantitative yield, which can then be protonated to give the neutral carborane nido-4,5-C2B6H10 (1) in good yield. Both of these nido electron-count clusters are shown to have an arachno-type geometry, i.e. a six-membered open face. The nido-4,5-C2B6H10 (1) hydroborated alkenes or alkynes which following deprotonation gave nido-7-R-4,5-C2B6H8- (2a(-)-c(-)) ions. Both nido-4,5-C2B6H9- (1(-)) and nido-4,5-C2B6H10 (1) serve as useful precursors to other adjacent cage-carbon clusters. Thus, nido-4,5-C2B6H9- (1(-)) reacted with BH3 . THF to give arachno-5,6-C2B7H12- (3(-)) which a single-crystal X-ray diffraction study showed is the first carborane to adopt the n-B9H15 cage geometry. Thermal or chemical degradation of nido-4,5-C2B6H10 (1) gave closo-2,3-C2B5H7 (5) in good to moderate yields. The nido-4,5-C2B6H9- (1(-)) was also found prone to lose a cage boron as evidenced by its reactions with (eta-C5H5)Co(CO)I-2 and (eta(6)-C(6)Me(6))(2)Ru2Cl4 which gave closo-3,1,2-(eta-C5H5)CoC2B5H7 (6) and closo-3,1,2-(eta(6)-C(6)Me(6))RuC2B5H7 (7), respectively. NMR studies showed the nido-4,5-C2B6H10 (1) was converted to arachno-4,5-C2B6H11- by reaction with LiEt(3)BH, and an alkyl derivative, arachno-7-CH3-4,5-C2B6H10- (4(-)), was formed by reacting MeLi with nido-4,5-C2B6H9- (1(-)) followed by protonation. The closo-2,3-C2B5H7 (5) was also converted in high yields to the smaller nido carborane, nido-2,3-C2B4H8, via reaction with TMEDA/H2O, and to nido-3,4-C2B5H8- (8(-)) by reaction with LiEt(3)BH.