Recently, Hirao and coworkers have reported the synthesis of cyclopentadienyl iron complexes of sumanene. From their experimental works, it was found that the concave-bound cyclopentadienyl iron complex of sumanene was only obtained in the crystal prepared. Conversely, theoretical calculations showed the convex-bound cyclopentadienyl iron complex of sumanene was relatively stable than the concave-bound one. Thus, there is a confliction between the experimental and theoretical research results. To solve this problem, the density functional theory calculations were carried out for the several types of cyclopentadienyl iron complexes with/without AlCl ion. These calculations suggest that AlCl ion plays an important role for the stabilization of the concave-bound cyclopentadienyl iron complex of sumanene. © 2012 Wiley Periodicals, Inc.