Bonding interactions in olefin (C2X4, X = H, F, Cl, Br, I, CN) iron tetracarbonyl complexes: Role of the deformation energy in bonding and reactivity

The iron-olefin bond energies for the monoolefin iron tetracarbonyl complex es Fe(CO)(4)(C2X4) (X = H, F, Cl, Br, I, CN) have been determined using den sity functional theory (DFT), with the BP86 functional. An energy decomposi tion analysis of the bonding interactions demonstrate that, as predicted by current models of metal-olefin bonding, the attractive electronic interact ions of the haloolefins and percyanoethylene with iron are stronger than th ose of ethylene. However, in addition to these electronic interactions the net bond energy depends on the energy needed to deform the Fe(CO)(4) and ol efin moieties from their equilibrium geometries to the geometrical conforma tion they adopt in the complex. This energy is termed the deformation energ y. As a result of the deformation energy, the bond energies for the substit uted olefins are similar to or smaller than that of the Fe-C2H4 bond. More than half of the total deformation energy involves deforming the olefin, pr incipally as a result of a chance in hybridization of the carbon atoms from sp(2) in the free olefin toward an sp(3)-like carbon in the bound olefin. The deformation of Fe(CO)(4) involves mainly the axial CO ligands, which be nd away from the olefin as a result of a repulsive interaction with the ole fin substituents. In addition, the increase in the C-X bond length, upon bo nding of the olefin to Fe(CO)(4), correlates well with the exothermicity of the oxidative addition reaction, Fe(CO)(4)(C2X4) --> XFe(CO)(4)(C2X3), ind icating that the deformation of the bound olefin lowers the energy of the C -X bond.