Treatment of [U(Cp*)₂Cl₂] with Na₂dddt in thf afforded the “ate” complex [U(Cp*)₂Cl(dddt)Na(thf)₂] (1), and the salt-free compound [U(Cp*)₂(dddt)] (2) could be extracted from 1 with toluene (Cp* = η-C₅Me₅; dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate). Reduction of 2 with Na(Hg) or addition of Na₂dddt to [U(Cp*)₂Cl₂Na(thf)_x] in the presence of 18-crown-6 gave the first uranium(III) dithiolene compound, [Na(18-crown-6)(thf)₂][U(Cp*)₂(dddt)] (4). The dimeric lanthanide complexes [{Ln(Cp*)₂(dddt)K(thf)₂}₂] (Ln = Ce (5), Nd (6)) were prepared by reaction of [Ln(Cp*)₂Cl₂K] with K₂dddt, and in the presence of 15-crown-5, they were transformed into the cation−anion pairs [K(15-crown-5)₂][Ln(Cp*)₂(dddt)] (Ln = Ce (7), Nd (8)). The crystal structures of 2, 4·thf, 5−7, 7·0.5(pentane), and 8·0.5(pentane) were determined by X-ray diffraction analysis. Comparison of the structural parameters of the anions [M(Cp*)₂(dddt)]^- (M = U, Ce, Nd) revealed that the U−S and U−C(Cp*) distances are shorter than those expected from a purely ionic bonding model; the relatively small folding of the dddt ligand suggests that the interaction between the CC double bond and the metal center is weak, in agreement with the NMR observations in solution. The structural data obtained from molecular geometry optimizations on the complexes [M(Cp*)₂(dddt)]^-,0 (M = Ce, U) using relativistic density functional theory (DFT) calculations reproduce experimental trends. A detailed orbital analysis shows that the contraction of the metal−sulfur bond lengths when passing from [Ce(Cp*)₂(dddt)]^- to [U(Cp*)₂(dddt)]^- is partly related to the uranium 5f orbital−ligand mixing, which is greater than the cerium 4f orbital−ligand mixing. The comparison of the two [U(Cp*)₂(dddt)]^-,0 species reveals a higher ligand-to-metal donation in the case of the U(IV) complex.