In this paper, density functional theory is used to investigate (benzene)chromium tricarbonyl, (cyclopentadienyl)manganese tricarbonyl, (1,3-butadiene)iron tricarbonyl, and (cyclopentadienyl)cobalt dicarbonyl. For the energies of low-lying temporary anion states, the stabilized Koopmans-based (S-KB) and stabilized Koopmans theorem (S-KT) methods are adopted. Stabilization is accomplished by varying the exponents of appropriate diffuse functions. Results indicate that the calculations of S-KB using PBEPBE and S-KT using CAM-B3LYP are able to yield energies of temporary anion states in good agreement with the experimental values. Furthermore, the ionization potentials call be determined accurately via the Koopmans-based (KB) PBEPBE method.