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 complexes Fe(CO)₄(C₂X₄) (X = H, F, Cl, Br, I, CN) have been determined using density functional theory (DFT), with the BP86 functional. An energy decomposition analysis of the bonding interactions demonstrate that, as predicted by current models of metal-olefin bonding, the attractive electronic interactions of the haloolefins and percyanoethylene with iron are stronger than those 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 olefin moieties from their equilibrium geometries to the geometrical conformation they adopt in the complex. This energy is termed the deformation energy. As a result of the deformation energy, the bond energies for the substituted 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, principally as a result of a change in hybridization of the carbon atoms from sp² in the free olefin toward an sp³-1ike carbon in the bound olefin. The deformation of Fe(CO)₄ involves mainly the axial CO ligands, which bend away from the olefin as a result of a repulsive interaction with the olefin substituents. In addition, the increase in the C-X bond length, upon bonding 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), indicating that the deformation of the bound olefin lowers the energy of the C-X bond.