Fe(CO) n (n=1–5) complexes have been studied using density functional theory (DFT) methods. Several functionals have been used in the geometry optimizations, harmonic frequencies computation and calculation of the iron–carbonyl bond dissociation energies. Coupled-cluster single double (triple) bond dissociation energies have also been computed for the smaller systems. The obtained results show that DFT methods yield reasonable geometries and vibrational frequencies. Regarding the bond dissociation energies, it is shown that the validity of the results depends on whether there is a change in the atomic state of the metal during the dissociation. When the atomic state is the same for both complexes, the bond dissociation energy computed using gradient corrected functionals is within the range of the experimental values, while when the atomic state changes, DFT methods overestimate the bond dissociation energy due to a poor description of the atomic multiplets.

Fe(CO) 4 PR 3 complexes have been studied for R ) H, Me, Ph, OMe, F, i-Pr, and NC 4 H 4 using density functional methods. The Fe-PR 3 bond has been analyzed in terms of steric and electronic effects. The results obtained show that the main contribution to the bond stems always from the σ donation, but phosphines can be classified into three groups depending on the relative magnitude of the π-back-donation contribution. Thus, PMe 3 , PPh 3 , and P(i-Pr) 3 can be considered as σ-donor ligands, PF 3 and P(NC 4 H 4 ) 3 would be σ-donor/πacceptor ligands, and PH 3 and P(OMe) 3 would correspond to intermediate cases.

Density functional theory calculations have been carried out on the trigonal complexes OsO3E and MCl3E (M = V, Ta) and the square pyramidal systems MCl4E (M = Cr, Mo, W, Re) for E = O, S, Se, and Te as well as (C5H5)ReO3. All complexes were fully optimized, and the calculated geometrical parameters are in reasonable agreement with gas-phase electron diffraction data where available. The calculated M−E bond energies decrease from oxygen to tellurium, from bottom to top in a metal triad, and from left to right in a transition series. The trend setting factor is the donation from the dσ metal orbital to the pσ acceptor orbital on the chalcogen atom. The contribution from the chalcogen to metal π back-donation has a maximum for sulfur and selenium. However in relative terms, the contribution from the π back-donation to the total M−E bond energy increases from oxygen to tellurium. Comparisons are made to previous calculations and experimental data on M−E bond strengths.

The structure and fluxional behavior of (η4-butadiene)Fe(CO)2L (L = CO, PH3, PMe3) complexes have been studied using density functional methods. For (butadiene)Fe(CO)3, the geometry obtained is in excellent agreement with the gas-phase experimental data. The calculation of the harmonic vibrational frequencies has permitted the reassignment of several frequencies observed in the IR and Raman spectra. The computed Fe−butadiene binding energy is in all cases about 52 kcal mol-1, in excellent agreement with the experimental data corresponding to the (butadiene)Fe(CO)3 complex. The nature of the bonding has been analyzed in terms of steric and electronic interactions. The butadiene−Fe rotational barriers have been computed, and the origin of the barrier has been discussed.