Inorganic Chemistry, Vol.41, No.25, 6646-6655, 2002
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
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; 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;