Theoretical Analysis of the Bonding between CO and Positively Charged Atoms

Abstract

A detailed analysis of the changes in the electronic structure of CO when a proton or a positive charge approaches the carbon or the oxygen atom is reported using quantum mechanical ab initio calculations and several methods to analyze the theoretical data. The C−O bond is shortened by nearly the same amount in HCO+ and QCO+ compared to free CO, while the nearly identical C−O bond lengths of COH+ and COQ+ are longer than in CO. H+ and Q+ have a strong electrostatic effect upon the atom to which they are bonded, which leads to an increased electronegativity of carbon and oxygen, respectively. Inspection of the charge distribution and the natural localized orbitals shows clearly that the shorter C−O distances of HCO+ and QCO+ and the longer C−O bond lengths of COH+ and COQ+ are due to the changes in the polarization of the bonding orbitals which are caused by the positive charge of H+ or Q+ that are bonded to the molecule. The bonding orbitals of CO are polarized toward the more electronegative oxygen end. A proton or a positive charge at carbon attracts electronic charge from the oxygen atom toward the carbon end, which leads to less polarized σ- and π-bonds and to a more covalent C−O bond. A positive charge or a proton at the oxygen atom has the opposite effect. The calculated curve of the C−O bond length in MCO+ (M = Li, Cu, Ag, Au) as a function of the M+−CO distance shows that the C−O bond becomes shorter in the beginning when the metal cation approaches the carbon atom. There is a turning point at shorter M+−CO distances where the C−O bond becomes longer again. The charge decomposition analysis shows that the position of the turning point is determined by the onset of the metal+ → CO back-donation. A relatively small amount of M+ → CO back-donation is sufficient to lengthen the CO bond. The turning point for the curve of the C−O bond length as a function of the M+−CO distance occurs at a M+−CO value that is shorter than the equilibrium distance for M = Li and Ag, while it is longer for M = Cu and Au. The trends of the bond strengths and M+−CO interactions are explained with the radii and orbital energies of the valence ns and (n − 1)d orbitals of the transition metals.

Size and charge effects on the binding of CO to late transition metal clusters

“…Higher level, basis set independent Hartree-Fock ab initio calculations contradict the molecular orbital results and show that the 5 orbital in free CO is, if anything, bonding with regard to the C-O coordinate. 22 Rather, the high values of ͑CO͒ in the nonclassical CO metal complexes could be shown to be due to electrostatic effects related to the cationic metal centers encountered in these complexes. 22,23 The general conclusion from these and other arguments is that M ← C bonding plays no major role in determining ͑CO͒ in transition metal carbonyls in general.…”

“…22 Rather, the high values of ͑CO͒ in the nonclassical CO metal complexes could be shown to be due to electrostatic effects related to the cationic metal centers encountered in these complexes. 22,23 The general conclusion from these and other arguments is that M ← C bonding plays no major role in determining ͑CO͒ in transition metal carbonyls in general. 23 Note that this does not mean that bonding is unimportant, but only that ͑CO͒ is relatively insensitive to it.…”

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.…”

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

“…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.…”