Dl. Cedeno et al., Bonding interactions in olefin (C2X4, X = H, F, Cl, Br, I, CN) iron tetracarbonyl complexes: Role of the deformation energy in bonding and reactivity, J PHYS CH A, 105(34), 2001, pp. 8077-8085
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.