Ismail, the benzene-Cr(CO)5 compounds are classical pi-arene complexes where the stable coordination mode of the arene is hexahapto. See Elschenbroich's "Organometallics" for a simplified MO treatment or E. L. Muetterties, J. R. Bleeke, E. J. Wucherer, T. Albright, Chem. Rev., 1982, 82, 499–525 for a more detailed treatment. It is a donation, backdonation type interaction where the former has the most contribution to bonding, and hence more electron rich arenes will donate more electron density to the metal and will bind stronger. If you are looking at the local minimum of the dihapto interaction, similar criteria would apply. The solid state structures for your chlorobenzene derivative and its fluorobenzene analog have been reported, and they have hexahapto coordination modes.

Thanks Roland so much for the quick answer.

According to what you said, haloarenes should bind stronger to the metal than benzene would do. but according to my calculations, the bond is weaker!

Pleasure. The inductive effect of halogens renders the haloarenes less electron rich than benzene, and hence less electron donating and weaker binding.