The electronic structure and chemical bonding of B-5- and B-5 were investigated using anion photoelectron spectroscopy and ab initio calculations. Vibrationally resolved photoelectron spectra were obtained for B-5- and were compared to theoretical calculations performed at various levels of theory. Extensive searches were carried out for the global minimum of B-5-, which was found to have a planar C-2v structure with a closed-shell ground state ((1)A(1)). Excellent agreement was observed between ab initio detachment energies and the experimental spectra, firmly establishing the ground-state structures for both B-5- and B-5. The chemical bonding in B-5- was investigated and compared to that in Al-5-. While both B-5- and Al-5- have a similar C-2v planar structure, their pi-bonding orbitals are different. In Al-5-, a pi-bonding orbital was previously observed to delocalize over only the three central atoms in the C-2v ground-state structure, whereas a similar pi orbital (1b(1)) was found to completely delocalize over all five atoms in the C-2v B-5-. This pi bonding in B-5- makes it more rigid towards butterfly out-of-plane distortions relative to Al-5(-).