The Koopmans-based (KB) approximation is used to investigate the ionization potentials of ferrocene and dibenzene chromium in density functional theory. As to the energies of low-lying temporary anion states of these transition metal complexes, the stabilization method coupled with KB approximation (S-KB) is adopted. Here, the stabilization is accomplished by varying the exponents of appropriate diffuse functions. Results indicate that the S-KB method is much more successful than other methods in predicting absolute and relative energies of temporary anion states. Furthermore, the ionization potentials via KB approach are very close to the experimental values.