The [B10H12](2-) dianion has been shown by the ab initio/IGLO/NMR method to have a C-2 symmetric structure (26) derived from B10H14 (17) by removing two opposite bridge protons. Adduct formation with one or two solvent molecules, suggested on the basis of experimental NMR investigations, does not take place. [B10H12. nL](2-) (n = 1, 2) structures with various ligands are not bound (vs [B10H12](2-) and n L) and do not reproduce the experimental B-11 NMR chemical shifts. The [B10H13](-) structure (19), computed to have C-1 rather than C-s symmetry in solution (as in the solid state), also can be derived from B10H14 (17) by removal of a bridging proton. In both the mono- (19) and the dianion (26), a bridging hydrogen can rearrange easily from B5/B6 to B9/B10 (barrier ca. 5 kcal mol(-1)) but not from B8/B9 to B9/B10 (barrier ca. 15 kcal mol(-1)). The recently proposed 6,6-(C5H5N)(2)B10H12 structure is not supported computationally.