Lanthanide Complexes Containing a Terminal LnIII-O Bond: Hydrolysis as a Tool to Assess f-Element Bond Covalency

We report the preparation, isolation, and reactivity of gas-phase lanthanide nitrate and acetate complexes featuring the elusive trivalent Ln(III)=O bond. Complexes [Ln(III)(O)(X)(2)](-) (X=NO3- or CH3CO2-; Ln=La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Tm, Lu) are prepared from [Ln(III)(CH3CO2)(X)(3)](-) precursors through decarboxylation followed by nitromethane or acetone elimination. The oxo complexes hydrolyze at rates indicating Ln(III)=O bond stability. The rates for [Ln(III)(O)(NO3)(2)](-) are essentially invariant, whereas those for [Ln(III)(O)(CH3CO2)(2)](-) exhibit a moderate decrease across the lanthanide series. The kinetics of lanthanide-oxo bond hydrolysis are assessed in the context of participation of 5d(2) electrons in bonding, changes in covalency via variations in 5d orbital energies and radial extensions, and steric crowding around the lanthanide center. The observed fast hydrolysis rates and lack of correlation to electronic and qualitative covalent considerations confirm the expected strong polarization in Ln(III)=O bonding, with variations in covalency minimally impacting reactivity. The Ln(III)=O bond reactivity is compared with previous results for Ln(III)-O & sdot; and Ln(IV)=O, and actinide An(III)=O and An(IV)=O; implications for lanthanide/actinide and lanthanide/lanthanide partitioning are discussed. Additionally, nitromethane and acetone elimination are demonstrated as useful for inducing a 2e(-) O-atom transfer resulting in non-oxidative formation of lanthanide-oxos.