AbstractAb initio electron propagator calculations on the electron affinities of the $ {\rm Be(\eta ^5-C_5H_5)}^{+} $ complex cation and electron detachment energies of the corresponding anion have been performed at the optimized geometry of the ground state of the neutral species. These results, obtained with a basis set that includes many diffuse and polarization functions, provide the relative energies of doublet states of $ {\rm Be(\eta ^5-C_5H_5)} $. In the former set of calculations, energy differences between the 2A1 ground‐state and various excited doublet states are associated with promotion of an unpaired electron from the 6a1 molecular orbital that consists chiefly of s functions on the Be nucleus to higher Be‐centered molecular orbitals that exhibit various patterns of interference between s, p, and d functions. Differences between electron detachment energies of the anion provide the energy separation between the radical's ground state and an excited state in which an electron is promoted from ligand‐centered π orbitals to 6a1. Dyson orbitals and pole strengths corresponding to correlated electron binding energies calculated with various diagonal self‐energy approximations confirm these qualitative pictures of the electronic structure of the lowest states of $ {\rm Be(\eta ^5-C_5H_5)} $. © 2012 Wiley Periodicals, Inc.