Equilibrium geometries, bond dissociation energies and relative energies of axial and equatorial iron tetracarbonyl complexes of the general type Fe(CO) 4 L (L = CO, CS, N2, NO + , CN ± , NC ± , g 2 -C 2 H 4 , g 2 -C 2 H 2 , CCH 2 , CH 2 , CF 2 , NH 3 , NF 3 , PH 3 , PF 3 , g 2 -H 2 ) are calculated in order to investigate whether or not the ligand site preference of these ligands correlates with the ratio of their r-donor/p-acceptor capabilities. Using density functional theory and effectivecore 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) 4 Fe±L bond dissociation energies (D 0 ) 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 ± , CH 2 and CCH 2 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 ± , CF 2 and CS. The Fe(CO) 4 L complexes with L = CO, g 2 -C 2 H 4 , g 2 -C 2 H 2 , NH 3 , PH 3 and PF 3 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 NF 3 , N 2 and g 2 -H 2 . A detailed examination of the (CO) 4 Fe±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) 4 L complexes with the r-donor/p-acceptor ratio of their respective ligands L thus does not generally support the classical picture of p-accepting ligands preferring equatorial coordination sites and r-donors tending to coordinate in axial positions. In particular, this is shown by iron tetracarbonyl complexes with L = g 2 -C 2 H 2 , g 2 -C 2 H 4 , g 2 -H 2 . Although these ligands are predicted by the CDA to be stronger r-donors than p-acceptors, the equatorial isomers of these complexes are more stable than their axial pendants.