X-ray Crystal Structure of Thorium Tetrahydroborate, Th(BH₄)₄, and Computational Studies of An(BH₄)₄ (An = Th, U)

The crystal structure of Th(BH₄)₄ is described. Two of the four BH₄^- ions are terminal and tridentate (κ³), whereas the other two bridge between neighboring Th^IV centers in a κ²,κ² (i.e., bis-bidentate) fashion. Thus, each thorium center is bound to six BH₄^- groups by 14 Th-H bonds. The six boron atoms describe a distorted octahedron in which the κ³-BH₄^- ions are mutually cis; the 14 ligating hydrogen atoms define a highly distorted bicapped hexagonal antiprism. The thorium centers are linked into a polymer consisting of interconnected helical chains wound about 4-fold screw axes. The structures of An(BH₄)₄ (An = Th, U) were also investigated by DFT. The geometries of [An(BH₄)₆]^2-, [An₃(BH₄)₁₆]^4-, and [An₅(BH₄)₂₆]^6- fragments of the polymeric structures were optimized at the B3LYP and/or PBE levels. Most calculated geometries are 14-coordinate and agree with the experimental structures, but isolated [Th(BH₄)₆]^2- units are predicted to feature 16-coordinate Th^IV centers.