Why Do Cationic Carbon Monoxide Complexes Have High C−O Stretching Force Constants and Short C−O Bonds? Electrostatic Effects, Not σ-Bonding

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.

NATURE OF Mδ+···δ+C-Oδ-INTERACTIONS IN METAL CARBONYLS. AN ELECTRONIC STUDY BASED ON THE TOPOLOGY OF THE ELECTRON CHARGE DENSITY DISTRIBUTION AND ITS LAPLACIAN FUNCTION

“…30 Estos valores son relativamente altos comparados con los valores de ρ(r b ) en interacciones débiles como enlaces de hidrógeno y enlaces de halógeno De acuerdo al teorema del virial local, las densidades de energía cinética, G(r b ), y potencial, V(r b ), están relacionadas por la expresión:…”

“…Sin embargo, algunos trabajos sugieren que las interacciones electrostáticas entre la especie metálica y la molécula de CO pueden proporcionar efectos adicionales interesantes en estos sistemas. Los cálculos sugieren que la carga de metal puede desempeñar un papel importante en la polarización de la densidad electrónica del CO. [30][31][32] Desafortunadamente, hay pocos estudios de carbonilos metálicos aislados en fase gaseosa en donde investigar estas predicciones. En este trabajo se estudia, entre otras cosas, como varia la polarizabilidad del para electrónico del C del CO y su influencia en la estabilidad de la especie metálica.…”

A DFT Study of CO Catalytic Oxidation by N2O or O2 on the Co3O4(110) Surface

“…It indicates that the highest occupied molecular orbital (HOMO) 5s of the CO molecule transfers electrons to the substrate. Since the 5s orbitals are slightly antibonding or essentially nonbonding, [41] 5s donation-dominated bonding should cause a slight blue-shift or have no intrinsic effect on the CO vibrational frequency. Does the p*-back-donation takes part in the bonding interactions?…”

Gold Cluster Carbonyls: Saturated Adsorption of CO on Gold Cluster Cations, Vibrational Spectroscopy, and Implications for Their Structures

“…This leads to a strengthening of the C-O bond and to a rising of the stretching force constant. 40 Au-CO bonding in AuCO + is thought to involve some degree of Au + f CO π-back-donation, despite the high value of ν(CO) (ranging from 2237 cm -1 in a neon matrix to ∼2000 cm -1 in AuCO + salts) that classifies the bonding as "nonclassical" under the criterion that ν(CO) is greater than its value in free CO. 41 It is recognized that, while π-back-donation weakens the CO bond and usually reduces ν(CO) to below that of free CO, its effect can be offset by the electrostatic effect of the charge on the metal interacting with the CO dipole. We attribute the decrease in ν(CO) with cluster size to charge dilution.…”