Why do cationic carbon monoxide complexes have high C-O stretching force constants and short C-O bonds? Electrostatic effects, not σ-bonding

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.