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 CC 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.

Treatment of [M(BH4)3(THF)3] with NEt3HBPh4 in THF afforded the cationic complexes [M(BH4)2(THF)5][BPh4] [M = U (1), Nd (2), Ce (3)] which were transformed into [M(BH4)2(18-crown-6)][BPh4] [M = U (4), Nd (5), Ce (6)] in the presence of 18-crown-6; [U(BH4)2(18-thiacrown-6)][BPh4] (7) was obtained from 1 and 18-thiacrown-6 in tetrahydro-thiophene. Compounds 1, 3.C4H8S, 4.THF, 5, and 6.THF exhibit a penta- or hexagonal bipyramidal crystal structure with the two terdentate borohydride ligands in apical positions; the BH4 groups in the crystals of 7.C4H8S are in relative cis positions, and the thiacrown-ether presents a saddle shape, with two diametrically opposite sulfur atoms bound to uranium in trans positions. The crystal structures of these complexes, as well as those of previously reported [M(BH4)2(THF)5]+ cations, do not reveal any clear-cut lanthanide(III)/actinide(III) differentiation. The structural data obtained for [M(BH4)2(18-crown-6)]+ (M = U, Ce) by relativistic density functional theory (DFT) calculations are indicative of a small shortening of the U...B with respect to the Ce...B distance, which is accompanied by a lengthening of the U-Hb bonds and an opening of the Hb-B-Hb angle (Hb = bridging hydrogen atom of the eta3-BH4 ligand). The Mulliken population analysis and the natural bond orbital analysis indicate that the BH4 -->M(III) donation is greater for M = U than for M = Ce, as well as the overlap population of the M-Hb bond, thus showing a better interaction between the uranium 5f orbitals and the Hb atoms. The more covalent character of the B-H-U three-center two-electron bond was confirmed by the molecular orbital (MO) analysis. Three MOs represent the pi bonding interactions between U(III) and the three Hb atoms with significant 6d and 5f orbital contributions. These MOs in the cerium(III) complex exhibit a much lesser metallic weight with practically no participation of the 4f orbitals.