Reactions of Lanthanide Atoms with Oxygen Difluoride
and the Role of the Ln Oxidation State
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Abstract
Laser-ablated
lanthanide metal atoms were condensed with OF<sub>2</sub> in excess
argon or neon at 4 K. New infrared absorption bands were observed
and assigned to the oxidative addition products OLnF<sub>2</sub> and
OLnF on the basis of <sup>18</sup>O isotopic substitution and electronic
structure calculations of the vibrational frequencies. The dominant
absorptions in the 500 cm<sup>–1</sup> 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 OCeF<sub>2</sub>, OPrF<sub>2</sub>, and OTbF<sub>2</sub> 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 OLnF<sub>2</sub> 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 OCeF<sub>2</sub>, 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 OLnF<sub>2</sub> compounds in the III oxidation state is predicted to be fully ionic.
The bonding in OLnF<sub>2</sub> 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 OLnF<sub>2</sub> analogues is
indicative of more ionic character in the OLnF species, consistent
with the more formal negative charge on the oxygen in OLnF