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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 , CCH 2 , CH 2 , CF 2 , NH 3 , NF 3 , PH 3 , PF 3 , η 2 ‐H 2 )
Author(s) -
Chen Yu,
Hartmann Michael,
Frenking Gernot
Publication year - 2001
Publication title -
zeitschrift für anorganische und allgemeine chemie
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.354
H-Index - 66
eISSN - 1521-3749
pISSN - 0044-2313
DOI - 10.1002/1521-3749(200105)627:5<985::aid-zaac985>3.0.co;2-#
Subject(s) - chemistry , bond dissociation energy , dissociation (chemistry) , basis set , ligand (biochemistry) , valence (chemistry) , bond energy , bond length , crystallography , density functional theory , computational chemistry , molecule , crystal structure , biochemistry , receptor , organic chemistry
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 – , η 2 ‐C 2 H 4 , η 2 ‐C 2 H 2 , CCH 2 , CH 2 , CF 2 , NH 3 , NF 3 , PH 3 , PF 3 , η 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 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) 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, η 2 ‐C 2 H 4 , η 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 η 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 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 = η 2 ‐C 2 H 2 , η 2 ‐C 2 H 4 , η 2 ‐H 2 . 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.