Reaction of U(NEt2)(4) with HS-2,4,6-(Bu3C6H2)-Bu-t (HSMes*) gave U(SMes*)(3)(NEt2)(py) (1), whereas similar treatment of U[N(SiMe3)SiMe2CH2][N(SiMe3)(2)](2) afforded U( SMes*)[ N( SiMe3) 2] 3 ( 2) and U(SMes*)(3)[N(SiMe3)(2)]. The first neutral homoleptic uranium(IV) thiolate to have been crystallographically characterized, U(SMes*)(4) (4), was isolated from the reaction of U(BH4)(4) and KSMes*. The first homoleptic thiolate complex of uranium(III), U(SMes*)(3) (5), was synthesized by protonolysis of U[N(SiMe3)(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 center dot center dot center dot H-C epsilon agostic interaction of each thiolate ligand is shorter, by similar to 0.05 angstrom, 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 center dot center dot center dot 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.