Ligand Site Preference in Iron Tetracarbonyl Complexes Fe(CO) ₄ L (L = CO, CS, N ₂ , NO ⁺ , CN ^– , NC ^– , η ² ‐C ₂ H ₄ , η ² ‐C ₂ H ₂

Abstract

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.

Assessing the Ligand Properties of NHC‐Stabilised Phosphorus(I) Cations

“…The metal centres in [LM(CO) 5 ][BPh 4 ] adopt a slightly distorted octahedral geometry and the iron atom in [LFe(CO) 4 ][BPh 4 ] adopts a trigonal bipyramidal geometry, whereby [L] + occupies an axial position. Some evidence has shown that σ‐donor ligands have a preference for axial site coordination in Fe(CO) 4 complexes and that strong π‐acceptors have a preference for the equatorial site (although there is some debate on the topic), suggesting that [L] + is most likely not an excellent π‐acceptor (as a comparison, the π‐acidic phosphenium cation (Et 2 N) 2 P + coordinates Fe(CO) 4 in the equatorial position) . The Fe−P and M−P bond lengths fall between those of zwitterionic triphosphenium–M(CO) 5 complexes (which are slightly shorter) and NHC‐phosphinidene‐adduct–M(CO) 5 complexes (which are slightly longer) .…”

Bimetallic Complexes from Amphoteric Group 13/15 Ligands: Syntheses and X-ray Crystal Structures

“…In agreement with the observations in solution, the phosphorus atom occupies an axial position, which is preferred by ligands with strong s-donor and weak pacceptor abilities. [19] As was found for the chromium complex 3 the PÀM' interaction (M' transition metal) in 5 seems rather weak, and this leads to the longest PÀFe(CO) 4 bond (237.7(1) pm) reported so far (observed range for pentacoordinated iron: 216 ± 237 pm). [9] For the related complex [{(Me 3 -Si) 3 P}Fe(CO) 4 ] a PÀFe bond length of 233.8(4) pm was found.…”

“…Yield: 0.10 g (0. 19 ] (0.5 mmol; E P: 0.20 g; E As: 0.22 g, E Sb: 0.25 g) in pentane (20 mL). A smooth gas evolution was observed, and the resulting suspension was slowly warmed to room temperature.…”

DFT studies of structural preference of coordinated ethylene in W(CO)3(PX3)2(CH2CH2) (X=H, CH3, F, Cl, Br, and I)

“…The increasing order of above-mentioned r-donation is understood in terms of the percent of polarized electron toward the W center in the W-P r-bond, which decreases in the following manner: PF 3 (%37.61) ) P(Me) 3 (%32.56) > PCl 3 (%30.50) > PH 3 (%30.18) > PBr 3 (%30.08) > PI 3 (%29.33). This is in accord with the recent calculation by Frenking and coworkers [18] which showed donation of phosphine ligand in Fe(CO) 4 PF 3 to be higher than donation in Fe(CO) 4 PH 3 .…”

“…These ligands are important because their electronic and steric properties can be altered in wide range by varying R. The phosphines, in addition to r-donor property (via a hybrid orbital containing a lone pair on phosphorus) are also considered as double-face p-acceptors (via empty r à orbitals of the P-R bonds) in which the nature of the R group determines the relative donor/acceptor ability [15]. Recently, the theoretical studies have also been done on complexes containing phosphine ligands [16][17][18].…”