Haptotropic rearrangements π-πin fluorenyl complexes are carried out using density functional theory DFT at PBE⁄TZP level. The calculated activation barriers in ŋ6 ŋ5 of (C13H9)ML complexes, (M=Mn, Cr and Fe), (L=(CO)3 and Cp) are (27.731, 63.922 and63.708 Kcal.mol-1 respectively)and in ŋ5 ŋ3of (ŋ5-9-R-C13H8)Mn(CO)3 complexes, R=But and Ph are 28.522 and 69.500 Kcal.mol-1 respectively).Thermodynamic parameters of these compounds have been analyzed. The calculated of the redox potentials and the quantum chemical descriptors predict that the iron complex has strong biological activity and strong Lewis acid. Keywords: Haptotropic rearrangements, DFT, activation barriers, redox potentials, biological