Reaction of U(NEt(2))(4) with HS-2,4,6-(t)Bu(3)C(6)H(2) (HSMes) gave U(SMes)(3)(NEt(2))(py) (1), whereas similar treatment of U[N(SiMe(3))SiMe(2)CH(2)][N(SiMe(3))(2)](2) afforded U(SMes)[N(SiMe(3))(2)](3) (2) and U(SMes)(3)[N(SiMe(3))(2)]. The first neutral homoleptic uranium(IV) thiolate to have been crystallographically characterized, U(SMes)(4) (4), was isolated from the reaction of U(BH(4))(4) and KSMes. The first homoleptic thiolate complex of uranium(III), U(SMes)(3) (5), was synthesized by protonolysis of U[N(SiMe(3))(2)](3) with HSMes in cyclohexane. The crystal structure of 5 exhibits the novel eta(3) ligation mode for the arylthiolate ligand. Comparison of the crystal structure of 5 with those of the isomorphous lanthanide congeners Ln(SMes)(3) (Ln = La, Ce, Pr, and Nd) indicates that the U-S, U-C(ipso)(), and U-C(ortho)() bond lengths are shorter than the corresponding ones in the 4f-element analogues, when taking into account the variation in the ionic radii of the metals. The distance between the uranium and the carbon atoms involved in the U...H-C epsilon agostic interaction of each thiolate ligand is shorter, by approximately 0.05 A, than that expected from a purely ionic bonding model. The lanthanide(III)/actinide(III) differentiation was analyzed by density functional theory (DFT). The nature of the M-S bond is shown to be ionic strongly polarized at the sulfur for M = U and iono-covalent (i.e. strongly ionic with low orbital interaction), for M = Ln. The strength of the U...H-C epsilon agostic interaction is proposed to be controlled by the maximization of the interaction between U(+) and S(-) under steric constraints. The eta(3) ligation mode of the arylthiolate ligand is also obtained from DFT.

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.