Abstract
A significant increase in the C-O stretching force constant (F(CO)) and a decrease in C-O bond length (r(CO)) result upon coordination of carbon monoxide to various cationic species. We report a study designed to elucidate the factors responsible for this effect. In particular, we distinguish between an explanation based on electrostatic effects and one based on withdrawal of electron density from the 5σ orbital of CO, an orbital generally considered to have some antibonding character. Ab initio electronic structure calculations on CO in the presence of a positive point charge (located on the carbon side of the bond axis) reveal that a simple Coulombic field increases the C-O stretching force constant and decreases the bond length. Coordination of CO to a simple cationic Lewis acid such as H+ or CH3 + is calculated to increase F(CO) (and decrease r(CO)) to extents slightly less than those engendered by a point charge at the same distance from the carbonyl carbon. These results indicate that electron donation from the 5σ orbital has no intrinsic positive effect on the magnitude of F(CO). Calculations were also conducted on several symmetrical, neutral, and cationic transition metal complexes, including some examples of the recently discovered homoleptic noble-metal carbonyls. It is found that F(CO) values can be quantitatively interpreted using a model which invokes only the effects of M-CO π-back-bonding and an electrostatic parameter. There is no correlation between the extent of σ-bonding (as measured by the depopulation of the CO σ orbitals) and F(CO). Calculations on trigonal bipyramidal d8 metal pentacarbonyls permit a comparison between inequivalent ligands (axial and equatorial) which, being coordinated to the same metal center, must experience approximately the same electrostatic field. In the case of Ru(CO)5, π-back-bonding to the axial and equatorial carbonyls is of virtually equal magnitude, while σ-donation is much greater from the axial ligands than from the equatorial ligands. Nevertheless, the F(CO) and r(CO) values of the two ligand sets are essentially equal, confirming that the magnitude of σ-donation does not affect these parameters.
| Original language | English |
|---|---|
| Pages (from-to) | 12159-12166 |
| Number of pages | 8 |
| Journal | Journal of the American Chemical Society |
| Volume | 118 |
| Issue number | 48 |
| DOIs | |
| Publication status | Published - 1996 |
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Why do cationic carbon monoxide complexes have high C-O stretching force constants and short C-O bonds? Electrostatic effects, not σ-bonding. / Goldman, Alan S; Krogh-Jespersen, Karsten.
In: Journal of the American Chemical Society, Vol. 118, No. 48, 1996, p. 12159-12166.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - Why do cationic carbon monoxide complexes have high C-O stretching force constants and short C-O bonds? Electrostatic effects, not σ-bonding
AU - Goldman, Alan S
AU - Krogh-Jespersen, Karsten
PY - 1996
Y1 - 1996
N2 - A significant increase in the C-O stretching force constant (F(CO)) and a decrease in C-O bond length (r(CO)) result upon coordination of carbon monoxide to various cationic species. We report a study designed to elucidate the factors responsible for this effect. In particular, we distinguish between an explanation based on electrostatic effects and one based on withdrawal of electron density from the 5σ orbital of CO, an orbital generally considered to have some antibonding character. Ab initio electronic structure calculations on CO in the presence of a positive point charge (located on the carbon side of the bond axis) reveal that a simple Coulombic field increases the C-O stretching force constant and decreases the bond length. Coordination of CO to a simple cationic Lewis acid such as H+ or CH3 + is calculated to increase F(CO) (and decrease r(CO)) to extents slightly less than those engendered by a point charge at the same distance from the carbonyl carbon. These results indicate that electron donation from the 5σ orbital has no intrinsic positive effect on the magnitude of F(CO). Calculations were also conducted on several symmetrical, neutral, and cationic transition metal complexes, including some examples of the recently discovered homoleptic noble-metal carbonyls. It is found that F(CO) values can be quantitatively interpreted using a model which invokes only the effects of M-CO π-back-bonding and an electrostatic parameter. There is no correlation between the extent of σ-bonding (as measured by the depopulation of the CO σ orbitals) and F(CO). Calculations on trigonal bipyramidal d8 metal pentacarbonyls permit a comparison between inequivalent ligands (axial and equatorial) which, being coordinated to the same metal center, must experience approximately the same electrostatic field. In the case of Ru(CO)5, π-back-bonding to the axial and equatorial carbonyls is of virtually equal magnitude, while σ-donation is much greater from the axial ligands than from the equatorial ligands. Nevertheless, the F(CO) and r(CO) values of the two ligand sets are essentially equal, confirming that the magnitude of σ-donation does not affect these parameters.
AB - A significant increase in the C-O stretching force constant (F(CO)) and a decrease in C-O bond length (r(CO)) result upon coordination of carbon monoxide to various cationic species. We report a study designed to elucidate the factors responsible for this effect. In particular, we distinguish between an explanation based on electrostatic effects and one based on withdrawal of electron density from the 5σ orbital of CO, an orbital generally considered to have some antibonding character. Ab initio electronic structure calculations on CO in the presence of a positive point charge (located on the carbon side of the bond axis) reveal that a simple Coulombic field increases the C-O stretching force constant and decreases the bond length. Coordination of CO to a simple cationic Lewis acid such as H+ or CH3 + is calculated to increase F(CO) (and decrease r(CO)) to extents slightly less than those engendered by a point charge at the same distance from the carbonyl carbon. These results indicate that electron donation from the 5σ orbital has no intrinsic positive effect on the magnitude of F(CO). Calculations were also conducted on several symmetrical, neutral, and cationic transition metal complexes, including some examples of the recently discovered homoleptic noble-metal carbonyls. It is found that F(CO) values can be quantitatively interpreted using a model which invokes only the effects of M-CO π-back-bonding and an electrostatic parameter. There is no correlation between the extent of σ-bonding (as measured by the depopulation of the CO σ orbitals) and F(CO). Calculations on trigonal bipyramidal d8 metal pentacarbonyls permit a comparison between inequivalent ligands (axial and equatorial) which, being coordinated to the same metal center, must experience approximately the same electrostatic field. In the case of Ru(CO)5, π-back-bonding to the axial and equatorial carbonyls is of virtually equal magnitude, while σ-donation is much greater from the axial ligands than from the equatorial ligands. Nevertheless, the F(CO) and r(CO) values of the two ligand sets are essentially equal, confirming that the magnitude of σ-donation does not affect these parameters.
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U2 - 10.1021/ja960876z
DO - 10.1021/ja960876z
M3 - Article
VL - 118
SP - 12159
EP - 12166
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
SN - 0002-7863
IS - 48
ER -