A density functional theory and wave function theory investigation on the geometrical and electronic structures of B5O40/- clusters has been performed in this work. B5O4- anion proves to possess a perfect tetrahedral ground state of T-d B(BO)(4)(-) ((1)A(1)) analogous to BH4-with four equivalent -BO terminals around the B center, while B5O4 neutral favors a slightly off-planed C-s B(BO)(4) ((2)A') which contains three -BO terminals and one -O- bridge. An intramolecular BO radical transfer occurs from T-d B(BO)(4)(-) to C-s B(BO)(4) when one electron is detached from the anion. The one-electron detachment energies of the anion and characteristic stretching vibrational frequencies of -B=O groups at about 2000 cm(-1) have been calculated to facilitate future experimental characterization of these clusters.