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

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 (De) 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 De(La) > De(Gd) > De(Lu) for the same isomer with different lanthanoid atom, and it is De(I) > De(III) > De(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 cha...