The new monocarbaborane dianion, arachno-4-CB8H122- has been synthesized from the reaction of arachno-4CB(8)H(14) with 2 equiv of NaH in polar solvents. DFT/GIAO computations at the B3LYP/6-311G*//B3LYP/6-311G* level, in conjunction with 1D and 2D NMR spectroscopic studies, have confirmed that the dianion results from deprotonation of both the endo-CH and one bridging hydrogen of the parent arachno-4-CB8H14. While the DFT calculations indicate that a G symmetric structure is lowest in energy, the experimental solution NMR data are consistent with the dianion having a C-s symmetric structure, thus suggesting that it is fluxional in solution. Transition state calculations located a low-energy pathway with an activation energy of only 2.7 kcal/mol that allows the migration of the bridging hydrogen between the two enantiomeric forms of the dianion. The process can occur by a single-step, simple rotation through a transition, state structure containing a -BH2 group at the B7 boron. Averaging the calculated B-11 NMR chemical shifts of the resonances for those atoms in the static enantiomeric structures that become equivalent by this fluxional process then gives excellent agreement with the solution NMR data. Transition state calculations of the fluxional behavior previously observed for the isoelectronic arachno-4-CB8H13- and arachno-4-SB8H11- monoanions have likewise revealed related low-energy (0.3 and 5.0 kcal/mol, respectively) rearrangement mechanisms involving the simultaneous rotation of three hydrogens (two bridging and one -BH2) through a C-s symmetry transition state containing three -BH2 groups.