Theoretical studies of organometallic compounds.

Equilibrium geometries, bond dissociation energies and relative energies of axial and equatorial iron tetracarbonyl complexes of the general type Fe(CO)₄L (L = CO, CS, N2, NO⁺, CN^-, NC^-, η²-C₂H₄, η²-C₂H₂, CCH₂, CH₂, CF₂, NH₃, NF₃, PH₃, PF₃, η²-H₂) are calculated in order to investigate whether or not the ligand site preference of these ligands correlates with the ratio of their σ-donor/π-acceptor capabilities. Using density functional theory and effective-core potentials with a valence basis set of DZP quality for iron and a 6-31G(d) all-electron basis set for the other elements gives theoretically predicted structural parameters that are in very good agreement with previous results and available experimental data. Improved estimates for the (CO)₄Fe-L bond dissociation energies (D₀) are obtained using the CCSD(T)/II//B3LYP/II combination of theoretical methods. The strongest Fe-L bonds are found for complexes involving NO⁺, CN^-, CH₂ and CCH₂ with bond dissociation energies of 105.1, 96.5, 87.4 and 83.8 kcal mol^-1, respectively. These values decrease to 78.6, 64.3 and 64.2 kcal mol^-1, respectively, for NC^-, CF₂ and CS. The Fe(CO)₄L complexes with L = CO, η²-C₂H₄, η²-C₂H₂, NH₃, PH₃ and PF₃ have even smaller bond dissociation energies ranging from 45.2 to 37.3 kcal mol^-1. Finally, the smallest bond dissociation energies of 23.5, 22.9 and 18.5 kcal mol^-1, respectively are found for the ligands NF₃, N₂ and η²-H₂. A detailed examination of the (CO)₄Fe-L bond in terms of a semi-quantitative Dewar-Chatt-Duncanson (DCD) model is presented on the basis of the CDA and NBO approach. The comparison of the relative energies between axial and equatorial isomers of the various Fe(CO)₄L complexes with the σ-donor/π-acceptor ratio of their respective ligands L thus does not generally support the classical picture of π-accepting ligands preferring equatorial coordination sites and σ-donors tending to coordinate in axial positions. In particular, this is shown by iron tetracarbonyl complexes with L = η²-C₂H₂, η²-C₂H₄, η²-H₂. Although these ligands are predicted by the CDA to be stronger σ-donors than π-acceptors, the equatorial isomers of these complexes are more stable than their axial pendants.