| Abstract
| - The molecular structures and equilibrium conformations of Zr(BH4)4 and U(BH4)4 have been determined by DFT calculations, gas electron diffraction, and matrix isolation IR spectroscopy.
- 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 theformer is confirmed, but both DFT calculations and GED structure refinements indicate that the BH4 groups are rotatedsome 12° away from the orientation in which the three bridging B−H bonds are staggered with respect to the opposingZrB3 fragment. As a result the symmetry of the equilibrium conformation is reduced from Td to T. Bond distances andvalence 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 Zranalogue. Matrix-isolation IR spectroscopy and DFT calculations on U(BH4)4 show that while the polymeric solid-statestructure is characterized by terminal triply bridging and metal−metal bridging bidentate BH4 groups, all BH4 groups aretriply bridging in the gaseous monomer. Calculations with one of the two nonbonding 5f electrons on U occupying an a1and 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 UB3fragment with τ(BUBHb) = 60°. Calculations including spin−orbit interactions indicate that Jahn−Teller distortions fromTd symmetry are either absent or very small. The best agreement between observed and calculated GED intensity datawas obtained for a model of Td symmetry, but models of T symmetry with dihedral angles τ(BUBHb) > 42° cannot beruled 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 thedifferent equilibrium conformations of the three molecules are determined primarily by repulsion between bridging Hatoms in different tetrahydroborate groups.
|
