Molecular Structures of Two Metal Tetrakis(tetrahydroborates), Zr(BH4)4and U(BH4)4: Equilibrium Conformations and Barriers to Internal Rotation of the Triply Bridging BH4Groups

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Abstract
The molecular structures of Zr[(μ-H)3BH]4 and U[(μ-H)3BH]4 have been investigated by density functional theory (DFT) calculations and gas electron diffraction (GED). The triply bridged bonding mode of the tetrahydroborate groups in the former is confirmed, but both DFT calculations and GED structure refinements indicate that the BH4 groups are rotated some 12° away from the orientation in which the three bridging B−H bonds are staggered with respect to the opposing ZrB3 fragment. As a result the symmetry of the equilibrium conformation is reduced from Td to T. Bond distances and valence angles are as follows (DFT/GED): Zr−B = 232.2/232.4(5) pm; Zr−Hb = 214.8/214.4(6) pm; B−Hb = 125.3/127.8(8) pm; B−Ht = 119.4/118.8(17) pm; ∠ZrBHb = 66.2/65.6(3)°; the smallest dihedral angle of type τ(BZrBHb) = 48/45(2)°. DFT calculations on Hf(BH4)4 indicate that the structure of this molecule is very similar to that of the Zr analogue. Matrix-isolation IR spectroscopy and DFT calculations on U(BH4)4 show that while the polymeric solid-state structure is characterized by terminal triply bridging and metal−metal bridging bidentate BH4 groups, all BH4 groups are triply bridging in the gaseous monomer. Calculations with one of the two nonbonding 5f electrons on U occupying an a1 and the other distributed equally among the three t2 orbitals indicate that the equilibrium conformation has Td symmetry, i.e. that the three B−Hb bonds of each tetrahydroborate group are exactly staggered with respect to the opposing UB3 fragment with τ(BUBHb) = 60°. Calculations including spin−orbit interactions indicate that Jahn−Teller distortions from Td symmetry are either absent or very small. The best agreement between observed and calculated GED intensity data was obtained for a model of Td symmetry, but models of T symmetry with dihedral angles τ(BUBHb) > 42° cannot be ruled out. Bond distances and valence angles are as follows (DFT/GED): U−B = 248.8/251.2(4) pm; U−Hb = 227.7/231.5(6) pm; B−Hb = 126.0/131.6(5) pm, B−Ht = 119.5/117.8(11) pm; ∠UBHb = 65.6/63.1(3)°. It is suggested that the different equilibrium conformations of the three molecules are determined primarily by repulsion between bridging H atoms in different tetrahydroborate groups.