Theoretical studies of organometallic compounds. 44 Ligand site preferencein iron tetracarbonyl complexes Fe(CO)(4)L (L = CO, CS, N-2, NO+, CN-, NC-, eta(2)-C2H4, eta(2)-C2H2, CCH2, CH2, CF2, NH3, NF3, PH3, PF3, eta(2)-H-2)

Equilibrium geometries, bond dissociation energies and relative energies of axial and equatorial iron tetracarbonyl complexes of the general type Fe(C O)(4)L (L = CO, CS, N2, NO+, CN-, NC-, eta (5)-C2H4.)eta (2)-C2H2, CCH2. CH 2, CF2NH3, NF3, PH3. PF3, eta (2)-H-2) are calculated in order to investiga te whether or not the ligand site preference of these ligands correlates wi th the ratio of their sigma -donor/pi -acceptor capabilities. Using density functional theory and effective-core potentials with a valence basis set o f 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 ve ry good agreement with previous results and available experimental data. Im proved estimates for the (CO)(4)Fe-L bond dissociation energies (Do) are ob tained using the CCSD(T)/II//B3LYP/II combination of theoretical methods. T he 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 mo l(-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)(4)L complexes with L=CO, eta (2)-C2H4, eta (2)-C2H2. NH3, PH3 and PF3 have even smaller bond dissoc iation energies ranging from 45.2 to 37.3 kcal mol(-1). Finally, the smalle st bond dissociation energies of 23.5, 22.9 and 18.5 kcal mol(-1). respecti vely are found for the ligands NF3, N-2 and eta (2)-H-2. A detailed examina tion of the (CO)(4)Fe-L bond in terms of a semi-quantitative Dewar-Chatt-Du ncanson (DCD) model is presented on the basis of the CDA and NBO approach. The comparison of the relative energies between axial and equatorial isomer s of the various Fe(CO)(4)L complexes with the sigma -donor/pi -acceptor ra tio of their respective ligands L thus does not generally support the class ical picture of pi -accepting ligands preferring equatorial coordination si tes and sigma -donors tending to coordinate in axial positions. In particul ar, this is shown by iron tetracarbonyl complexes with L = eta (2)-C2H2, et a (2)-C2H4. eta (2)-H-2 Although these ligands are predicted by the CDA to be stronger sigma -donors than pi -acceptors, the equatorial isomers of the se complexes are more stable than their axial pendants.