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Molecular structures of two metal tetrakis(tetrahydroborates), Zr(BH4)(4) and U(BH4)(4): Equilibrium conformations and barriers to internal rotation of the triply bridging BH4 groups.
6646 - 6655.
The molecular structures of Zr[(mu-H)(3)BH](4) and U[(mu-H)(3)BH](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 12degrees 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 T-d to T Bond distances and valence angles are as follows (DFT/GED): Zr-B = 232.2/232.4(5) pm; Zr-H-b = 214.8/214.4(6) pm: B-H-b = 125.3/127.8(8) pm; B-H-t = 119.4/118.8(17) pm; <ZrBHb = 66.2/65.6(3)degrees; the smallest dihedral angle of type tau(BZrBHb) = 48/45(2)degrees. DFF 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 a, and the other distributed equally among the three t(2) orbitals indicate that the equilibrium conformation has T-d symmetry, i.e. that the three B-H-b bonds of each tetrahydroborate group are exactly staggered with respect to the opposing UB3 fragment with tau(BUBHb) = 60degrees. Calculations including spin-orbit interactions indicate that Jahn-Teller distortions from T-d 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 tau(BUBHb) > 42degrees cannot be ruled out. Bond distances and valence angles are as follows (DFT/GED): U-B = 248.8/251.2(4) pm; U-H-b = 227.7/231.5(6) pm; B-H-b = 126.0/131.6(5) pm, B-H-t = 119.511117.8(11) pm; <UBHb = 65.6/63.1(3)degrees. 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.
|Title:||Molecular structures of two metal tetrakis(tetrahydroborates), Zr(BH4)(4) and U(BH4)(4): Equilibrium conformations and barriers to internal rotation of the triply bridging BH4 groups|
|Keywords:||GAS ELECTRON-DIFFRACTION, CRYSTAL NEUTRON-DIFFRACTION, VIBRATIONAL-SPECTRA, TRANSITION-METAL, URANIUM BOROHYDRIDE, TETRAHYDROBORATE, APPROXIMATION, ENERGIES, HF(BH4)4, ELEMENTS|
|UCL classification:||UCL > School of BEAMS > Faculty of Maths and Physical Sciences > Chemistry|
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