Lanthanide(III) and Actinide(III) Complexes [M(BH4)2(THF)5][BPh4] and [M(BH4)2(18-crown-6)][BPh4] (M = Nd, Ce, U): Synthesis, Crystal Structure, and Density Functional Theory Investigation of the Covalent Contribution to Metal-Borohydride Bonding

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 tetrahydrothiophene. Compounds 1, 3 center dot C4H8S, 4 center dot THF, 5, and 6 center dot 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 center dot 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 center dot center dot center dot B with respect to the Ce center dot center dot center dot B distance, which is accompanied by a lengthening of the U-H-b bonds and an opening of the H-b-B-H-b angle (H-b = bridging hydrogen atom of the eta(3)-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-H-b bond, thus showing a better interaction between the uranium 5f orbitals and the H-b 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 T bonding interactions between U(III) and the three H-b 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.