Linkage Isomerism and the Relativistic Effect in Interaction of Lanthanoid and Carbon Monoxide

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 101; no. 49; pp. 9314 - 9317
• Main Authors: Hong, Gongyi, Lin, Xianjie, Li, Lemin, Xu, Guangxian
• Format: Journal Article
• Language: English
• Published: American Chemical Society 04.12.1997
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp970718+

The interaction between La, Gd, or Lu atoms and carbon monoxide has been studied by means of the density functional method with gradient correction and quasi-relativistic correction. Three linkage isomers, Ln−CO (I), Ln−OC (II), and Ln−(η2-CO) (III) have been obtained through full geometric optimization. The vibrational frequencies and the dissociation energies (D e) for decomposition into Ln atoms and CO have been calculated. The calculated frequency related to the C−O bond (νC - O) of Gd−CO is in good agreement with the experimental value. The ordering of the dissociation energy is D e(La) > D e(Gd) > D e(Lu) for the same isomer with different lanthanoid atom, and it is D e(I) > D e(III) > D e(II) for different isomers with the same lanthanoid. However, isomer III has much lower νC - O than does isomer I, showing that the traditional assumption used to estimate the binding strength between the metal and CO by the lowering of νC - O is not always valid. For La−CO and Gd−CO, the relativistic effect produces little changes in the calculated bond lengths and vibrational frequencies, but it significantly reduces the dissociation energy. For Lu−CO the relativistic effect lowers the spin multiplicity of the molecule in the ground state, significantly enlarges the bond lengths between Lu and CO, reduces the dissociation energy, and raises the νC - O. It is found that the bond formation can be described as the donation of the CO 5σ electrons to the Ln 5dσ orbitals in synergy with a feedback of the Ln 5dπ electrons to the CO 2π* orbitals. For isomer II the CO 5σ donation is very weak, and for isomer III both the CO 5σ and 1π electrons participate in donation. The donation of the bonding CO 1π electrons to Ln 5d orbitals may be the major cause for the large lowering of νC - O of isomer III. Accompanying the bond formation, there is transfer of roughly one electron from Ln 6s to 5d orbitals. With the increase of the Ln atomic number, the energy gap between the Ln 6s and 5d levels enlarges rapidly, leading to decrease of the bonding energy for the heavier lanthanoid−carbon monoxide compounds. The relativistic effect lowers the 6s level and raises the 5d level and thus enhances this trend.