The geometrical structures of [B3H8](-) and [ClCuB3H8](-) compounds have been investigated by means of ab initio calculations, using the MP2 level of theory, and density functional theory (DFT) method. The [ClCuB3H8](-) (3a) complex structure, in which two boron atoms were bound to the copper atom via two B-H-Cu bridge hydrogen bonds (one bridge bond for each boron), was the most stable. Its MP2 and DFT calculated geometry is comparable to that of B4H10. In addition, the DFT calculated vibrational frequencies are in good agreement with experimental values. On the other hand, the most favorable interconversion of 3a structure was found. to proceed with a low activation barrier (6.5 kcal mol(-1)). This is consistent with the NMR spectra and confirms the rapid fluxional behavior for this complex. Moreover, the isomerization leading to 3a, from the less stable structure 3c, also proceeds with a low energy barrier (5.2 kcal mol(-1)), whereas the activation barrier of reverse rearrangement is negligible. The calculated energy barriers of the rearrangement between the less stable structures 3b and 3c are also very small (1.2 kcal mol(-1)).