Ion pairing and solvation effects in the intra-5f2 absorption spectra of dimeric uranium(IV) alkoxide complexes

A spectroscopic study is presented for the [U[sub 2](OBu[sup t])[sub 9]][sup [minus]] dimer anion in a series of structurally similar compounds, including a TBA[sup +] salt reported and characterized here for the first time. Electronic absorption spectra are shown to reveal subtle manifestations of the influence of the coordination sphere on the metal-centered 5f[sup 2] manifold of electronic states. In crystalline KU[sub 2](OBu[sup t])[sub 9] each potassium is known to be closely associated with a single dimer anion. The spectra show this association to be maintained in nonaqueous solution. Weakening the association by increasing the size of the cation is shown to alter the f-f spectra by shifting bands to higher energy. Changing the solvent results in smaller shifts which are observable only at low temperature and are inferred to result from partial solvation of the cation. Quantitative trends in both the solvent-dependent and cation-dependent shifts are interpreted in terms of the influence of the environment on the electron repulsion within the 5f shell and on the crystal-field splitting of the free-ion states. 21 refs., 8 figs.