Density functional theory (DFT) calculations using BP86 and B3LYP* methods have been applied on a series of isoelectronic (CpFeCp')ML3, (CpFeP5)ML3, (CpFeCp')FeCb and (CpFeP5)FeCb (Cb = Cyclobutadiene, ML3 = Cr (CO)3, Mo(CO)3, CrBz, MBz, MnCp and Bz = Benzene) triple-decker complexes of 30-MVE (metal valence electrons) count to investigate the electronic structure and the bonding and exploiting the energy decomposition analysis (EDA). The optimized structures for the considered complexes are comparable to those experimentally characterized by X-ray diffraction. The bonding is evaluated through the interactions between the (cyclo-P5- ) or Cp- and ML3 metallic fragments pinpointing the a-donor and & pi;-acceptor performance of the (cyclo-P5- ) and Cpligands as potential 6 electron donors on one hand and that of the metallic fragments of 12-MVE count on other hand. The natural charges and the localization of the molecular orbitals have been used to predict the variation of the interactions with the nature of the metal fragment. The electrostatic and orbital interaction contributions and the role of a-donor and & pi; acceptor into the bonding are for a huge importance with regard to the energy decomposition analysis (EDA). For both (cyclo-P5- ) and Cp- , the bonding was revealed to be equally ionic and covalent related to the percentage of & UDelta;Eelstat and & UDelta;Eorb into the total of attractive interactions (& UDelta;Eelstat + & UDelta;Eorb). The & pi; bonding is strong than a one, which contributes significantly by about 75% and more into the total orbital interactions.The a-donation (Cp & RARR; M and P5 & RARR; M) and & pi;-backdonation (Cp & LARR; M and P5 & LARR; M) are evaluated based on the donor-acceptor electrons transfers, revealing that are more significant for P5- than those for Cp- .
