Reactions of Lanthanide Atoms with Oxygen Difluoride and the Role of the Ln Oxidation State

Abstract

Laser-ablated lanthanide metal atoms were condensed with OF₂ in excess argon or neon at 4 K. New infrared absorption bands were observed and assigned to the oxidative addition products OLnF₂ and OLnF on the basis of ¹⁸O isotopic substitution and electronic structure calculations of the vibrational frequencies. The dominant absorptions in the 500 cm^–1 region are identified as Ln–F stretching modes, which follow the lanthanide contraction. The Ln–O stretching frequency is an important measure of the oxidation states of the Ln and oxygen and the spin state of the complex. The OCeF₂, OPrF₂, and OTbF₂ molecules have higher frequency Ln–O stretching modes. The Ce is assigned to the IV oxidation state and the Pr and Tb are assigned to a mixed III/IV oxidation state. The remaining OLnF₂ compounds have lower Ln–O stretches, and the Ln is in the III oxidation state and the O is in the −1 oxidation state. For all of the OLnF compounds, the metal is in the III oxidation state, and the Ln–F bonds are ionic. In OCeF₂, OLaF, and OLuF, the bonding between the Ln and O is best described as a highly polarized σ bond and two pseudo π bonds formed by donation from the two 2p lone pairs on the O to the Ln. Bonding for the OLnF₂ compounds in the III oxidation state is predicted to be fully ionic. The bonding in OLnF₂ and OLnF is dominated by the oxidation state on the lanthanide and the spin state of the molecule. The observation of larger neon to argon matrix shifts for Ln–O modes in several OLnF molecules as compared to their OLnF₂ analogues is indicative of more ionic character in the OLnF species, consistent with the more formal negative charge on the oxygen in OLnF